Learn Multi platform Z80 Assembly Programming... With Vampires!
Platform Specific Lessons
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Platform Specific Series - Lets learn how the hardware of the systems work, so we can get it to do what we want... Covers Amsrad CPC,MSX,ZX Spectrum, TI-83,Enterprise 128/64 and Sam Coupe!
    Lesson P1 - Basic Firmware Text functions

Lesson P2 - More Text Functions, Improvements... and the Sam Coupe!

Lesson P3 - Bitmap graphics on the Amstrad CPC and Enterprise 128

Lesson P4 - Bitmap graphics on the ZX Spectrum and Sam Coupe

Lesson P5 - Bitmap graphics on the TI-83 and MSX

Lesson P6 - Keyreading on the Amstrad CPC, ZX Spectrum and Sam Coupe

Lesson P7 - Keyreading on the MSX, Enterprise and TI-83

Lesson P8 - Tilemap graphics on the Sega Master System & Game Gear

Lesson P9 - Tilemap graphics on the Gameboy and Gameboy Color

Lesson P10 - Tilemap graphics on the MSX1

Lesson P11 - Tilemap graphics on the MSX2

Lesson P12 - Joypad reading on Master System,GameGear, Gameboy and Gameboy Color

Lesson P13 - Palette definitions on the Amstrad CPC and CPC+

Lesson P14 - Palette definitions on the Enterprise and Sam Coupe

Lesson P15 - Palette definitions on the MSX2 and V9990

Lesson P16 - Palette definitions on the Sega Master System and Game Gear

Lesson P17 - Palette definitions on the Gameboy and Gameboy Color

Lesson P18 - Making Sound with the AY-3-8910 on the Amstrad CPC, MSX,ZX Spectrum.... and NeoGeo + Atari ST!!

Lesson P19 - Sound on the Elan Enterprise

Lesson P20 - Sound on the Sam Coupe

Lesson P21 - Sound on the Gameboy and GBC

Lesson P22 - Sound with the SN76489 on the Master System, GameGear, Megadrive (Genesis) and BBC Micro!

Lesson P23 - Sound with the 'Beeper' on the ZX Spectrum and Apple II

Lesson P24 - Bankswitching and hardware detection on the Amstrad CPC

Lesson P25 - Bankswitching and hardware detection on the MSX

Lesson P26 - Bankswitching and hardware detection on the ZX Spectrum

Lesson P27 - Bankswitching and hardware detection on the Enterprise

Lesson P28 - Bankswitching and hardware detection on the Sam Coupe

Lesson P29 - Hardware detection and Bank Switching on the Gameboy/GBC and Sega Mastersystem/GameGear

Lesson P30 - Hardware Sprites on the gameboy

Lesson P31 - Hardware Sprites on the Master System / Game Gear and MSX1!

Lesson P32 - Hardware Sprites on the CPC+

Lesson P33 - Bitmap Graphics on the Camputers Lynx

Lesson P34 - Sound and Keyboard on the Camputers Lynx

Lesson P35 - Playing Digital Sound with WAV on the AY-3-8910!

Lesson P36 - Playing Digital Sound with WAV on the CPC+ via DMA!

Lesson P37 - Playing Digital Sound with WAV on the Sam Coupe, Camputers Lynx and ZX Spectrum

Lesson P38 - Playing Digital Sound with WAV on the Sega MasterSystem/GameGear, Elan Enterprise and GameBoy/GBC

Lesson P39 - Setting the CPC screen with CRTC registers

Lesson P40 - Syncronized mode switches for 320x200 @ 16 color EGX graphics on the Amstrad CPC

Lesson P41 - CRTC Rupture for Interrupt based splitscreen on the CPC

Lesson P42 - Advanced CRTC Rupture

Lesson P43- ULANext on the Spectrum NEXT

Lesson P44- Enhancements to the Classic ULA and Low Res Mode (Radasjimian)

Lesson P45 - 256 color mode on the Elan Enterprise

Lesson P46- Tilemap on the Spectrum NEXT

Lesson P47- Using 16 color Mode 0 to simulate 2x 4 color Layers

Lesson P48 - All MSX2 Bitmap Commands - Part 1/2

Lesson P49 - All MSX2 Bitmap Commands - Part 2/2

Lesson P50 - Alternative Bitmap modes... HighRes, 256 color, YJK (MSX2+) and Interlaced!

Lesson P51 - Window - Tilemap Scrolling - Alt Tile Pattern addresses and Interrupts

Lesson P52 - MSX1 mode G2 for 768 onscreen tiles

Lesson P53 - Realtime Sprite Flipping on the Amstrad CPC

Lesson P54 - Transparency on Amstrad CPC software sprites

Lesson P55 - LightGun Reading on the Sega Master System

Lesson P56 - Pixel Plotting on the Amstrad CPC

Lesson P57 - Stereoscopic 3D on the SegaMasterSystem with the Segascope 3D Glasses

Lesson P58 - Modes 4, 3, 2 and 1 on the Sam coupe

Lesson P59 - Hardware scrolling on the MSX 1/2/2+

Lesson P60 - Tape loading on the Amstrad CPC (5K subs special)

Lesson P61 - 2x and 4x sprite scaling with Lookup Tables on the Amstrad CPC

Lesson P62 - AMX Mouse reading on the Amstrad CPC

Lesson P63 - Kempson Mouse reading on the ZX Spectrum + SpecNEXT

Lesson P41 - CRTC Rupture for Interrupt based splitscreen
We're going to take things to another level, and learn how to exploit the CRTC to do clever tricks...

First we'll take a look at rupture, which allows for CPC Hardware Splitscreen.


The CRTC in detail... Know your Enemy!
We've looked at the CRTC before, but we've not looked at it in enough depth for what we're going to do now... we need to look at things in a bit more detail!

Secrets of the CPC screen
We all know the 'Bitmap Screen' area of the CPC screen where our text and gameplay occur... and we all know too well the 'Border' area which doesn't really do much... but there's some more areas we need to know about...

As the beam of the CRT draws, the 'Offscreen area' is areas that are not visible on the screen, above, below or to the sides of the screen.

the Hblank area is to the right of the visible screen, the Vblank area is at the bottom...

The screen is divided into 8 pixel tall character blocks... so the total screen (including invisible areas) is 39 characters tall

Interrupts ALWAYS occur every 52 pixel lines, so they don't divide equally to character blocks... 

While an interrupt happens every 52 lines, we CAN trick the interrupt into happening earlier using a trick called RVI (Rupture: Vertical, invisible)

This combines multiple lines into one, and makes the interrupt happen faster, but it's VERY complex to get working, and we can't look at it yet.

CRTC Registers
The CRTC has 18 registers we can read and write to, but it also has some INTERNAL registers that we' can't change, but we can see them within WinApe, and we'll need to know what many of them do when it comes to using complex CRTC tricks

It's also important to note that some CRTC chips have limitations that others do not... we need to try to make sure our code works the same on ALL CRTC chips

Reg Num Abbrev Name Range Bits Default Value 256x192 384x272(26k) Details CRTC Limitations
&00 HTOT Horizontal Total 0-255 DDDDDDDD 63 (&3F) 63 63 Physical width of screen -1 � Leave alone!
&01 HDISP Horizontal Displayed 0-255 DDDDDDDD 40 (&28) 32 48 Logical width in Chars (8 pixels in mode 1)
&02 HSYNC Horizontal Sync Position 0-255 DDDDDDDD 46 (&2E) 42 51 Logical Xpos 2: HsncPos+HsyncWid must be < Htot
&03 V/HWID Horiz. and Vert. Sync Widths 0-15,0-15 VVVVHHHH 142 (&8E) 134 (8,6) 142 (8,14) Hsync / Vsync area size � Leave alone! 1+2: Vhei fixed to 16
&04 VTOT Vertical Total 0-127 -DDDDDDD 38 (&26) 38 38 Physical height of screen -1 � Leave alone!
&05 VADJ Vertical Total Adjust 0-31 ---DDDDD 0 0 0 Scanline Offset
&06 VDISP Vertical Displayed 0-127 -DDDDDDD 25 24 34 Logical Height in Chars (8 Pixels)
&07 VSYNC Vertical Sync position 0-127 -DDDDDDD 30 31 35 Logical Ypos of screen
Interlace and Skew 0-3 ------DD 0 0 0 0/2=off 1/3=on � Leave alone!
&09 MR Maximum Raster Address 0-31 ---DDDDD 7 7 7 Scanlines per Char row � Leave alone!
Cursor Start Raster 0-127 -DDDDDDD 0 0 0 unused
Cursor End Raster 0-31 ---DDDDD 0 0 0 unused
&0C DISPH Display Start Address 0-63 xxPPSSOO 00 / 16 /
32 / 48
00 / 16 /
32 / 48
12+1 / 28 /
44+1 / 60
PP=Screen Page (11=C000)
S=Size(11=32k else 16k) O=Offset

&0D DISPL Display Start Address 0-255 OOOOOOOO 0 0 0 O=Offset
&0E CURH Cursor Address 0-63 --DDDDDD 0 0 0 unused
&0F CURL Cursor Address 0-255 DDDDDDDD 0 0 0 unused
&10 LPH Light Pen Address 0-63 --DDDDDD 0 0 0 Read Only
&11 LPL Light Pen Address 0-255 DDDDDDDD 0 0 0 Read Only
internal VCC Vertical Character Count

Ypos of drawn line
internal R52 Raster 52 interrupt timer

Time since last interrupt
internal HDC Horizontal Display Counter

Current Hpos
internal HCC Horizontal Character Count

Xpos of drawn line
internal VMA Video Memory Address

Address of current screenpos
internal VLC Vertical Line Count

Line of character block being drawn
internal VSC Vertical Sync Counter

Winape emulator function � Time synce vsync
internal VTAC Vertical Total Adjust Counter

Time since end of frame
internal HSC Horizontal Sync Counter

Cycles during Hsync
internal VDUR VDU Raster

Winape Emulator function � offset within display

We can see the internal registers within Winape

Select Registers from the Debug menu

We can also turn on ROW HIGHLIGHT - this will show a line on the screen when the emulator is paused, which shows what the beam is currently drawing

Remember, the Memory address and screen size are only read in at the start of a new screen, and some tricks (like mode changes and RVI) need to be synchronized to the end of the line
We'll need to keep an eye on HCC and VCC to do this, and ROW HIGHLIGHT will help make things more clear.

Registers and how they affect the screen.
The position of the screen, the borders, and the Vblank locations are all controlled by the registers,

The current location of the beam is stored in the internal registers...

The physical size of the screen is fixed, and there are limits to what the CRTC can do within the screen... but there are some tricks we can do to exploit the CRTC and get it to do things it usually wouldn't!

If we mess with Hblank or Vblank areas, or VSYNC there's a very good chance we're going to end up with an unstable screen!
This can also happen if our screen width (HTOTAL) is too big,

This is just part of the difficulty of these CRTC tricks, we're going to have to do some of the work of the CRTC ourselves if we want to do this kind of thing.

Rupture... two screens are better than one!

We can change colors at any point during the screen, to get more than 4 colors on a Mode 1 screen (Like in ChibiAkumas)
We saw last week that EGX switches the screen mode every line to simulate a new screen mode...

But what if we want to have two areas of the screen that hardware scroll independently? we'd probably want to change registers R12 and R13 to set the display base... but unfortunately the CRTC only reads in this address when it starts drawing the screen...

But we can Cheat!... we define a short screen... then as soon as it finishes, define a new screen.... this will trick the CRTC into starting again... re reading in all the registers, and starting a whole new screen... this is known as Rupture (also known as Horizontal Rupture)

Unfortunately it's not so easy, we're going to have to trick the CRTC, and do a lot of the work of laying out the screen for it... so that we get the screen we want!

Lets take a look!

Horizontal Rupture... the theory.
There's a few steps we need to do to get Rupture working!

1. Synchronize with the screen redraw... We need to be very precise in our timing and change the screen layout during the redraw procedure
2. Set the VSYNC position (R7) to 255... this is impossible but we need to stop VSYNC occurring while we're messing... we're going to fire the VSYNC manually later once we're done

3. Set the height  of the screen in VTOT (R6)
4. Set the Memory address of the screen (R12+R13)
5. Wait for the beam to get to the bottom of the screen
6. Repeat from stage 3 for the next screen

7. Once all the screens are done, Set the VSYNC position (R7) to 0...
this forces the VSYNC to occur.
8. Wait for the screen redraw to start again... repeat from step 1

We're going to have to repeat this procedure every single frame the CPC draws - this is why Rupture is such a pain!

The Rules!
1.The screen must be a total of 312 lines otherwise the screen won't synchronize

2.Our timing must be precise or we're going to make a mess!

3.A new screen cannot start before an old one ends.

4.Our screen height will be R6 * (R9+1).... R9 is the character height (usually 8)... we can change this, but setting it to 0 will cause problems on some screens - it's best left alone at this stage.
We can only set the base address of the screen (in R12,R13) to an address from &C000-&C7FF... the other areas will be used for following lines within a character block.

A 'Simple' Example!
We're going to create a simple example... it will split the screen into 5 sub-screens! (one per interrupt)

Each screen can have base memory address... and a different screen mode!... though please note defining screen modes will slow down your game (just rem them out if you don't need them)

Because the interrupts are handling the stability of the Rupture effect, you won't need to worry about timing, just don't disable interrupts!

The example code will show some changing text, and also effect horizontal and vertical scrolling!

The Code
We're going to need to set things up for the Rupture,

We going to use self modifying code to change the interrupt handler every time an interrupt occurs...

We load a Jump, and the first interrupt address... Interrupt1

We now turn on interrupts, and start our code!
Interrupt 1...

At the start of each interrupt, we self-modify the jump, so the next interrupt will call Interrupt2 instead

We now have some code which will fake the top and bottom borders... as we are defining the screen ourselves, the borders only exists because of this code..

We now need to wait until the screen reaches the VSYNC state,  we do this by checking Bit 0 of port &F5xx.
We now wait a little while for the VSYNC area to finish, and the beam to start scanning the actual screen... The first part will be the Off-Screen area

First we want to set the height of the 'screen' (The first rupture section), we use Reg 4 to do this... we're going to set this to 4... this results in a screen 5 character blocks tall.... giving a screen with a total of 5x8= 40 pixel tall screen

We're now going to set the High and Low Memory addresses using Reg 12 and Reg 13... the format is a little odd... effectively, we need to bit shift the top byte by 2 bits, so if we want our screen to start at &C000 we need to set Reg 12 to &30 (&C0 shifted right twice)...

Note that we're using the settings of R2_Addr here... we're always defining the settings for the NEXT strip during the CURRENT interrupt

Finally we're going to set the Vsync position in Reg 7 to 255... this effectively means VSYNC cannot occur, we'll re-enable it after our last rupture
Our newly defined screen will only start being drawn after the current one completes...

The Interrupt happens slightly before this, which is great for our splitscreen, but if we want to change Screenmode for the block, then it won't line up with the rupture...

Unfortunately the only way we can fix this is by making screen mode changes, then waiting in NOPs... this wastes our CPU power, but it DOES allow us to align the rupture and screen mode change.

Once again, if we're simulating screen borders, we now want to set a proper screen size using Reg 6

Now We've done the first interrupt, we've covered all the complex stuff... the other 5 interrupts are just using the same kind of commands to repeat the procedure for another 4/5 'screens'...
Having so many screens is probably excessive, you'd probably only want 2 in reality... one for your status, and one for the game - or one for player 1, and one for player 2!

Each interrupt sets the screen settings for the FOLLOWING interrupt block, so Interrupt 1 sets 2's mode and address... so if you want to reprogram things you'll need to bear this in mind...

Interrupt 2-5...
Interrupts 2 to 5 are all the same, and just a simpler version of what we just saw.

Like before, We switch in our new interrupt handler,

Next we define the new screen height with Reg 4

Now ew  set the new Screen memory address with Reg 12+13

Finally we wait, then change our screen mode if needed....

Note... there is NO messing with borders, or VSYNC position this time... those were just for Interrupt handler 1!
Interrupt 6...
Time for the last interrupt!... it's a little different.

We need to define the next interrupt handler, this time we're using Interrupt 1

If we want borders, then we want a border to appear , so we define it with Reg 6

As before we define the height of the block with Reg 4.... the Screen memory address with Reg 12+13... we've seen this in Interrupt 2-5

The next bit is different... Now we need that VSYNC to occur... we want to make the Vsync occur now, so we set Reg 7 to Zero...

The VSYNC will occur, then the settings we just set will be used for the top of the new screen

Different CRTC versions have different limitations, and some don't like things other's are OK with... This example should work ok on all of them...

Lesson P42 - Advanced CRTC Rupture 
We used Rupture last time to do split-screen, but it's possible to use Rupture along with other registers to build a visible screen that is constructed from

Last time all our blocks were the typical 8 pixels tall, This time we're going to work with 4 pixel strips instead by altering Reg 9


Rupture of less than 8 pixels
Many parameters such as Screen address can only change every screen...but a screen can be any height... even one single character block, and as we can even change the height of character blocks we can have whatever we want,

In this example we'll use 4 character blocks just 1 pixel tall!

We'll use 3sets of 4 lines to build up the entire screen - a full overscan screen is being made from just a few hundred bytes of ram!
3 sets of 4 lines used to build screen
While we can set the screen base for the start of each screen, we can only set the base aligned to one of the original 8 pixel character blocks... this limits us to addresses like  &C000, &C800, &D000, &D800, &E000, &E800, &F000, &F800

Of course similar screen bases exist for ranges &0000-3FFF, &4000-7FFF and &8000-BFFF
Resulting screen
We can change the base by 1 character block (effectively a 2 byte horizontal scroll) but due to the limitations of R12 and R13, it's impossible to start a screen from one line down (for example &C050 cannot be the start of a screen)
CRTC Registers:

7 6 5 4 3 2 1 0
Reg 12 � DispH - - P P S S O O
Reg 13 � DispL O O O O O O O O
P=Page  S=Screen-size (16k/32k) O=Offset

Resulting Screen Address:

Screen Address P P L L L O O O
O O O O O O O -
P=Page L=Line in Charblock (Automatic - Cannot be set) O=Offset

Because we can only set the start of the screen to a limited number of memory locations, it can be difficult to get the screen to do exactly what we want with R12 and R13...
There is a solution... RVI (Rupture, Vertical - Invisible)... this creates a mini hidden screen at the start of a line a few pixels wide... the visible screen then starts on the next line - effectively manipluating the 3 bits that connot be set with R12+R13

We're going to set the Hsync width as small as we can - this reduces the 'black areas' of the off-screen parts - and will help stop 'black areas' when we have an excessive offset

Normal Hwid
Reduced Hwid
We need to ensure interrupts are controlled, when an interrupt occurs we're just going to immediately return,

This program is very time sensitive, so our interrupt handler just returns as quickly as possible
We need to sync exactly with the screen, we're going to ensure we're consistently aligned to the interrupts by checking port &F5 and waiting for a new screen to start.

The Main Loop
When our loop starts, we need to wait again for the first Vsync of a new screen...

When the new screen starts, we set the Vsync point to 255 - this is impossible, effectively ensuring that a Vsync cannot occur during our drawing of the screen
We need to wait a little while for the end of the Vsync,
We will then start defining our physical screen
We are going to define some rather odd screens!

Each 'character block' is just one line tall (defined by setting Reg 09 to &00)...

Each 'Screen' is just 4 character blocks tall (defined by setting Reg 04 to &03)...

This has the effect of making each screen just 4 pixels tall, and allows us to change screen address every 4 lines!... because we disabled Vsync, we can start a new screen after each one of these tiny screens!
We're going to use a 'Command list' for the main loop,this list contains entries, one for every 4 pixel tall screen (so 280 lines in total)

The command list contains 4 bytes...

The first two are the address of the 4 pixel tall screen (loaded into Reg 12 and 13)... the next is the Hsync position (Reg 2) - effectively this moves the horizontal position of the 4 pixel strip onscreen

The final byte is a screen mode - we can change screen mode every 4 pixels if we wish!

During the main loop, we load each of the 3 screen parameters, and OUT them to the video registers...

If we 're changing the screen mode, we then Out this to the gate array...

Once this is done, we perform a delay until the next 4 pixel screen starts... we define a large number of NOP commands (Byte &00) with a DS command.

We repeat this whole procedure for every 4 pixel screen.
Once the screen has finished drawing, we need to reset the screen settings so we can cause a Vsync.

Once we've done this, a vsync will occur, and when it does we can start the procedure again!

This may seem pretty logical, but the trouble is the timing of the 4 pixel tall screen creation has to be EXACT, and it takes all the CPU time.

If we were making a game, we'd effectively have to fit the entire game-play code into the NOP sections

Lesson P43- ULANext on the Spectrum NEXT
The ULA (Uncommitted logic array) is responsible for the color graphics on the spectrum, giving a choice of 7 colors in 2 brightness for foreground and background, it gives reasonable color for limited memory...

Unfortunately the high contrast palette makes the color clash a bit severe!... however the Spectrum NEXT offers an alternative... ULA NEXT!


In this example we'll look at the options of ULA NEXT, this allows the bits of the color attributes (&5800-&5AFF) to be reconfigured... alternatively we can just alter NEXT colors 0-31 to change the default colors the regular ULA gives.

ULA - The options!
Here's our Chibiakumas default screen... this is using the classic spectrum coloring options!

ULA NEXT Recolor... 3 bit Fore / 4 bit Back (actually 5bit - 1 unused) ULANEXT Alt ColorMap... 4 bit Fore / 4 bit Back
This version has been recolored! 3 bits have been used for the foreground color, and 4 bits for the background...
We can now select any color from the 256 color Spectrum Next palette

the result in this case is nicer colors, but the 'dark' colors have been lost, as we're only using 3 bits for the background... but the bitmap data is unchanged, so we can easily support ULA+ and classic spectrum with the same bitmap file
This version has been totally recolored... 4 bits used for foreground and background - meaning the color information is NOT the same as the classic spectrum...

We can now use 16 colors for the foreground - and if we want, a totally different 16 for the background...

Note: This screen was a crude test... a far better screen could be made with more time and work!

Spectrum NEXT registers used in this example
We're going to need a variety of Spectrum NEXT registers in today's example...

Bits  Default 
&E3 Sets the "transparent" colour for Layer 2, ULA and LoRes pixel data. (in RGB)
&40 R W PPPPPPPP &00 Palette select (0-7=inks , 8-15 Bright, 16-23: paper, 24-31 Bright paper)
&41 R W RRRGGGBB &FF Change Palette RGB
&42 R W MMMMMMMM &0F ULA Next Mode mask (Applied to color attrib... result is INK entry (umasked bits +128 for PAPER)
&43 R W IPPPSLUN &10 Palette option P=palette for rw S=sprite L=layer 2 U=ula N=ulanext
&00 Define palette in 9 bits / P=Priority in layer 2

The Key to ULANext is &42 This defines a set of rightmost bits (Eg %00000111 or %00011111) to define the INK color number - which is taken from the NEXT Palette... the remaining bits will be used for the Background color... which is taken from NEXT palette entries 128+

We're going to use a command called NextReg a lot in this example... it's a special Z80 opcode added to the processor, we've created a macro to allow our assembler to support it.

It compiles to db &ED,&91,\reg,\val

Let's use the NEXT ULA!
In this example, we're going to use the ChibiAkumas screen shown above, it's been converted to a binary SCR file 7K in size...

We're also going to use an alternative set of Color Attributes - 768 bytes in size these fill the &5800-&5AFF... and will replace the classic color attributes with a special one - allocating 4 bits to foreground colors, and 4 bits to background - for a 16 color palette...

These files were created with my free open source AkuSprite editor
Standard Spectrum Screen:
First we're going to copy the Screen bitmap over the visible screen (showing the image)

We'll set the border to black, then we'll pause so the user can see the image.

ULANext Relcolored Standard Spectrum Screen:
OK, we're going to make some changes - firstly we'll change the Transparent color with NextReg $14- normally it's bright magenta, but we may want to use magenta in our palette, so we'll change it to a different color we won't need

Next we need to enable ULA Next... we do this with bit 0 of NextReg $43

Now we need to tell the hardware how to use the Color attribute map... we'll set the Low 3 bits to the Foreground - which defines the remaining bits as the background
We've allocated 3 bits to our foreground - so we need 8 colors for our foreground palette (at palette entry 0)

We've allocated 5 bits to our foreground - our picture uses 8 background colors plus the Bright bit (flashing bit is unused... 4 used bits) - so we need 16 colors for our background palette (at palette entry 16)

We select the palette entry with NextReg $40 and use a DefinePalettes function to set the colors

The enhanced version will have nicer color, but we have no dark foreground colors due to the 3 bit foreground palette
We've defined a 1 byte per color palette in the format %RRRGGGBB... we define it by writing color bytes to NextReg $41
ULANext 4 bit per color Dedicated ColorMap Spectrum Screen:
We're going improve the colors using a special ULAnext set of color attributes... we copy the new ULA color info over the normal one in screen memory

This will result in a total mess on an classic spectrum!
Lets fix that mess!

we need to reprogram NextReg $42 with the new rules!.... 4 of the bits are our foreground definition... 4 are the background

We need to write the new palette... it's 16 colors, and we've used the same colors for the foreground and the background... we use NextReg $40 to select Palette entry 0 for the foreground, and Palette entry 128  for the background

This time we're using 9 bits per color %RRRGGGBB B so we use an alternative Palette definition routine (DefinePalettes9bit)
When we use 9 bit palettes, we write two bytes per color to NextReg $44... the 9th bit - the low blue bit - is in Bit 0 of the second byte.

The finished screen can be seen here!

Having completely configurable 16 colors for foreground and background adds a lot of potential, but we're still stuck with the design limitations of 2 colors per 8x8 block.

Still, if we want to support classic Spectrums in the same code, it's an easy way of giving the game a little improvement without too much extra work.

Lesson P44- Enhancements to the Classic ULA and Low Res Mode (Radasjimian)
Last time we looked at the new ULA NEXT option on the Spectrum NEXT, and used it to try some alternate Color Attribute configurations.
We do have an alternate option... Recolor the classic ULA palette by changing the first 32 Spectrum NEXT colors... the NEXT also adds a 'Low Res Mode' - allowing 256 colors at 128*96


Recolored Classic ULA
Classic ULA Recolored ULA
The Classic ULA Colors as seen on the Standard Spectrum We've used an alternate 16 color palette for the Dark & Bright colors...
We could have used a different palette for the foreground and background,
but in this example we've used the same palette for both!
Recoloring the ULA in this way works far better, and is far easier than the ULA Next option we looked at last time...

In fact, after seeing this effect, there's not a lot of benefit to the ULANext option - especially considering the loss of backwards compatibility

The Source Code
on the NEXT The Classic ULA will use the first colors of the Next Palette

Colors 0-15 will be the Dark and Bright Foreground Colors.... Colors 16-31 are the Dark and Bright background colors... we select the first color to change with NextReg &40

If we don't want Magenta to be the transparent color, we probably want to set that to a different color with Reg &14
Our palette setting code is the same as last time...

if we're using 1 byte per color, we write to NextReg &41

if we're using 2 bytes per color (9 bit), we write to NextReg &44
We can hide parts of the ULA with NextReg &1A... four writes to this address will define a window, hiding the rest of the ULA

According to the documentation, we should be able to use NextReg &32 and &33 to define an 'Offset' allowing the ULA to scroll... it works with LowRes mode, but not the ULA... so maybe there's a bug in the Emulator?

The ULA offset didn't work as expected when the Author of these tutorials tried to make it work... maybe he's to dumb to understand it, or maybe it's an emulator bug or mistake in the documentation...

But fear not!... When things become clearer, this documentation will be updated!

Radasjimian / Low Res Mode

Low Res mode is a linear format, each line is 128 bytes wide (128 pixels)... and each line is 128...  each pixel is 256 color and takes a single byte... so to move down a line we just add 128
The Screen memory is split into two separate areas of standard Ram:
Top Half
0-47 &4000-&57FF
Bottom Half
96-9 &6000-&77FF

To the right you can see a re-scaled version of the MSX chibiakumas screen - though it's only 16 color.

The Source Code
Using Lo Res mode is really easy! All we do is set the top bit of NextReg &15 - this enables Lo Res Mode...

It uses two &1800 byte large banks of ram at &4000 and &6000... so to copy our bitmap we do two LDIR commands
We set up colors for the LowRes Mode in the same way as usual on the Next.... in the case of the sample screen, it only uses 16 colors
We can use Next register &1A to define a 'window' - this will hide parts of the lowres screen
We can see the result below:
We can scroll the window with the registers &32 and &33, these set the offset of the layer

Fonts and Screen Position
When we want to write data to the screen, we can use the GetScreenPos function... We pass a (X,Y) co-ordinate in BC, and it returns an address in HL

Because the screen is 128 bytes wide, our formula is:

Address= ScreenBase+Ypos*128+Xpos

if the Ypos is <48 then the Screenbase is &4000, if it's 48 or over then the screenbase is &6000... we move to the &6000 area by adding &8 to the top byte
If we want to move down a screen line, all we do is add 128 to the current memory position.
Lets create a PrintChar command - to show the Accumulator from our bitmap font!

We'll use the 1bpp font we used in the main tutorials... it has no characters below character 32, and uses 8 bytes per character...

We subtract 32 from A, and multiply it by 8 to calculate the offset in our font of the character we want...

Next we load the X,Y pos of the 'cursor' from address CursorX and CursorY... each character is 8x8 pixels, so we multiply both by 8, and use GetScreenPos to calculate the address the character should be printed.
We need to convert our 1bpp font into 8bpp screen bytes... we do this by reading each byte of our font from DE,

Our Screen address is in HL... we back this up with a PUSH HL

We shift a bit off the left of the byte, and put it into a screen byte... we repeat this 8 times (with B as a counter)... we've done one line...

Now we need to do the next line... we restore the old address with POP HL... and call our GetNextLine function we wrote

We now use IXL as a counter for the 8 lines of our font.
We've printed the character, but we want to increase CursorX so the next character will be printed to the right of the last...

we also need to check if we've reached the end of the line, and move down a line with the NewLine function
The font is crazy big, but you can see the result here!

for practical purposes, you'd probably want a 5x5 font or something like that.

Lowres mode is... well, Low res! but it gives an extra 256 color layer - so you could use it for a parallax effect with Layer 2...

We'll look at options for combining layers in a later episode!

Lesson P45 - 256 color mode on the Elan Enterprise
The Enterprise mimics Mode 1 and 0 of the Amstrad CPC... these provide 320x200 with 4 colors or 160x200 at 16 colors...

But the Enterprise offers an extra mode... 80x200 at 256 color!

Using 256 Color mode (Mode 3)
To change the screen mode we need to alter bits 5,6  of the 2nd byte of the LPT... we need to set these to '3' which will define the 256 color mode.
In this mode the 'palette' in the LPT has no effect, and every byte of the screen ram IS a color definition of the pixel - in the same format as the palette used. Pixel bytes:
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
G0 R0 B0 G1 R1 B1 G2 R2
We can see the results to the right!

We can export a valid bitmap with the Akusprite Editor, just change the view mode to "ENT 80x200 256 color" and use the Save Raw Bitmap option

We're going to learn how to use a 4 pixel wide 'half nibble' font on the Enterprise..

This tutorial is based on the 'Simple Hello World' Episode, and we won't cover again the bits that are unchanged... if you've not seen that episode, please see it Here

4 pixel wide font for low resolution screens and saving RAM
Because our screen is pretty narrow, our normal 8 pixel wide font would just allow 10 characters per line of the screen!.

Instead we'll use just one nibble (4 bits) per character, halving the total size of our font, down to just 384 bytes for our 96 character font!

We call our PrintChar routine to draw a character to the screen...

Our font has two characters per byte... so when we work out the byte address in our font for the character, we first ignore the low bit... we then shift A two times to the left, effectively multiplying it by 4

We then add the base address of our Btimap font... the result is DE now contains the address of our character in the font.
We need to calculate the screen address... we use CursorX and CursorY bytes to store the next character position...

We multiply CursorX by 4 - as our font is 4 pixels wide... and CursorY
OK, we need to draw our character...

We use IXL as a counter for the 8 lines of the font... we load a line from the font into C from DE

Next we need to get back the character number we wanted to print - and see if it's odd or even...

If it's Even, we print the left most 4 bits (the high nibble %XXXX----)
If it's Odd, we print the right most 4 bits (the low nibble %----XXXX)... we do this by bit shifting left 4 times with 4x SLL C before each line...

To convert 1bpp to 8bpp, we shift one bit out of C into A, and show A to the screen, we repeat this 4 times to show the 4 pixel wide font...

We use GetNextLine to move our destination HL address in screen memory down a line, and repeat for the next line of the font.
Once we've printed the character, we move the CursorX for the next character print.

We've used the font here for a half width effect, but if you're really hard up for ram on a system like the Camputers Lynx, you could double up the bits in the font, for a 'chunky' 8x8 font, and save 384 bytes of font memory.

Lesson P46- Tilemap on the Spectrum NEXT
The Spectrum Next isn't just capable of Bitmap layers, it can also do Tilemaps... This allows us to create fast scrolling graphics using the ULA layer (still allowing Layer 2 for other purposes!)... Lets make the layer work!


Setting up the Tilemap
First we need to set up the palette... We select the tilemap palette... then use the DefinePalettes function we wrote before.
We need to turn the Tilemap on with bit 7 of reg &6B - we can also specify the size of the tilemap with bit 6 - we're setting it to 40x32

Bit 5 controls Attributes - when this is 0 each tile is defined by two bytes (the first is tilenumber, and the second being flip and palette option)... when it's 1 each tile is just one byte - the tilenumber... we'll allow attributes!
We need to define the address of the tilemap with &6D, and tile pattern data with &6E

Each uses 6 of the 8 bits - the top two are ignored - the Tilemap is always in page 5 of ram.

We're defining the tilemap at &4000, and the pattern data at &5000

Defining Tile patterns
Ok! we need to copy our Tilepattern data into the correct memory address... We do this with a LDIR

The tile pattern data is included from file.

You can export the correct Tile data with my Akusprite Editor - it's free and open source.

Select "Save Raw 4bpp 8x8 Tiles" From the Z80->SpecNext Menu

Filling the tilemap
We define the 'TileMap' which will select the tile number to show in each position on the screen.

We're going to need to copy this to the correct Ram area in screen.

With Attributes enabled, Entries in the tilemap take 2 bytes in the format below:

Byte 1  
Byte 2 P P P P X Y R U

N = Tile Number / U = ULA over Tilemap (or N bit 9) / R = Rotate Tile / Y = Y-Flip / X = X-Flip / P = Palette
When it come to filling the tilemap, we need to read from our 'Tilemap' label (shown above), and copy the bytes to &4000+

Our tilemap is 40x32, so we set BC to 1280 (&500)

We use IXL to set the second attribute tile for each byte... really this is just so we can test attributes in this example..

After writing each tilenumber we write IXL - we can filip some of the bits of IXL after each write if we want to test the attributes!
The tilemap will be visible onscreen.

Bit 0 of the attribute can either be 'ULA over tilemap' (Transparency) or bit 8 of a tilenumber (for 512 tile)... it just depends on the setting of NextReg &6B

Remember the Tilemap can only be in Page 5 (with the regular screen) - so if we using the Tilemap AND ULA, we're going to have too little memory to worry about losing the extra 256 tiles!
Scrolling and sizing the tilemap
Like with other layers, we can define a 'Window' to change the visible area of the tilemap...

By default the Tilemap is bigger than the classic 256x192 ULA, but we can resize it to match the normal speccy screen
We can also scroll the tilemap with Nextreg's &2F,&30 and &31
We can scroll and scale the tilemap

The Tilemap allows us to have good color graphics, and change and redraw things more quickly than a bitmap layer - so is a great choice for background graphics in our game - with sprites for our characters...

We can also use Layer 2 as... well a Layer 2(!) for parallax effect.

Lesson P47- Using 16 color Mode 0 to simulate 2x 4 color Layers
Some games from the 80's were known to use a trick where the 4 bits of color depth in Mode 0 were split into pairs... effectively giving 2 layers of 4 colors...

By ANDing and ORing new data in - sprites could change, without removing the background behind!... Lets check it out!


We're looking at today's example on the CPC, but the exact same technique will work fine on any system with 16 colors, and fully configurable palette.

The Enterprise has 16 colors, however as the 2nd 8 are copies of the first 8, it's unlikely this trick would work on that system.

Working with multiple Layers - The theory
This concept seems a little odd, and it's probably not the first one you want to consider for having sprites over background graphics, but it's at least worth knowing about!
This technique was used in Ghosts n Goblins and Bubble Bobble back in the 80's
We're splitting the color bits in 2 - so the result is 4 background colors and 4 (possibly different) foreground colors.

Our base Colors for Layer 1 will be Colors 0,1,2,3.... For Layer 2 they will be Colors 0,4,8,12
Colors where the foreground and background overlap (Colors 5,6,7,9,10,11,13,14,15) will need to be set to the same color as the foreground layer color (Though we can effect transparency by using a different color! See Later!)
Layer Color
 Layer Color
0 00 00 0 0
1 00 01 0 1
2 00 10 0 2
3 00 11 0 3
4 01 00 1 0
5 01 01 1 1
6 01 10 1 2
7 01 11 1 3
8 10 00 2 0
9 10 01 2 1
10 10 10 2 2
11 10 11 2 3
12 11 00 3 0
13 11 01 3 1
14 11 10 3 2
15 11 11 3 3

ScreenMode 0 uses a weird combination of bits, Each byte has two pixels -  the 8 bits split into two pairs - 4 for the left pixel, and 4 for the right... we'll need to split them into a 'Layer 1' and a 'Layer 2':

Bits   7   6 
Left Pixel 7
Right Pixel
Layer 1 7 6

3 2

Layer 2

5 4

1 0

Sprite Drawing Code
We're going to need a pair of sprite drawing routines - one for each layer.

Here's the version for Layer 1 (the background layer using colors 0-3)

Before writing data to the screen We're going to need to mask the current screen data, keeping bits %00110011 (the bits for layer 2) -

And then OR in new new sprite data for layer 1... we AND the sprite data with %11001100 to make sure there's no extra data in the sprite... you can skip this if your sprite only has 4 colors.

The code here uses the firmware for screen bitmaps functions.

Thre's an alternate version for the foreground layer - the bitmask for the background has been changed to %11001100 (Keep Layer 1)

We have to bitshift the sprite data 2 bits to the right, to convert the bitmap for the Layer 2 colors (0,4,8,12)... we mask the bits with %00110011 to remove any unwanted colors from the sprite.
We can use these functions to draw our sprite to the screen... we set DE to the Xpos, HL to the Ypos and BC to the sprite data we want to show.
When the two sprites we draw overlap - with color 0 transparent, the 2nd layer (using colors 0,4,8,12)  will take priority, because of the color palette we've used

If we clear the layer 2 sprite (by writing a blank sprite) ... the layer 1 sprite will be intact!

The same is true for layer 1
If we draw a sprite to Layer 2 and it overlaps an existing sprite, the layer 2 sprite will be erased!

The same is true for layer 1
if we set the Overlap Colors 5,6,7,9,10,11,13,14,15 (The colors made when the two layers overlap) to a different color - we can effectively make the layers transparent!

In this example, we've used a palette that will make an 'Anaglyph 3d effect' (Red-Cyan glasses)

Layer 1 is Red shades... Layer 2 is Blue/Cyan shades... The Overlap colors are Greys and White (Cyan=Green+Blue.... Red+Green+Blue=White)

Losing half your screen resolution is a high price to pay, but if you want nice sprites and backgrounds, and don't want to spare memory on any kind of screen buffers, it can give you an 'easy' way of moving sprites round the screen, without having to worry about redrawing your background.

And it's a bit of fun, don't you think?

Lesson P48 - All MSX2 Bitmap Commands - Part 1/2
The MSX V9938 VDP has a wide range of 'Commands'... these will make the VDP perform a variety of graphics task... depending on the command, we need to set different registers, and in some cases we need to check the status registers too...

Lets try each command and learn how they work


Direct Memory access with port &98
Accessing memory directly allows us to write or read bytes consecutively into VRAM...

We're going to use the 'SetReadAddress' and 'SetWriteAddress' to select a VRAM memory address.

The address is specified by registers AHL for addresses from &00000-&1FFFF... 128k

We have to do some bitshifting, and set bit 6 of byte 3 to either 1 or 0, for Read or Write... we write the result the the control port (&99)
Once we've selected an address, we can read or write to ram from port &98 (depending on bit 6 of the third byte we sent to the control port)

We can use this to define a read or write function to transfer bytes from consecutive addresses in VRAM
In this example, we'll transfer part of our game code to VRAM, then transfer it back to &D000 as a test
The results can be seen here our code will appear as corruption (we can see our green sprite in the middle)

If we dump &D000 to screen, we can see it contains the same data as &803A

You can use direct RAM access while the VDP is drawing other things... ChibiAkumas used this to draw the gradient background while the VDP was clearing the blank areas.

You can even use VRAM to bank out bits of your program code, then bank them back in later if you have spare VRAM but too little RAM... for example you could page out the disk settings and firmwaer ram at &E000-&FFFF to VRAM during your game, and bring it back when you load the next lever.

Common Commands

We're going to need some ports to work with the VDP, and we'll define them with symbols to make our code easier
Many of our functions can take a 'direction'... We're generally best going left->right,top->bottom, but in cases where there is an overlap (when we're copying a range to itself - eg a scroll effect) the order we copy will affect the result... suppose we copy (x,y)->(x+1,y)... if we work left to right - each line will be filled with the first pixel... if we reverse the direction - working right to left, the screen will be scrolled.
Some of our commands allow 'Logical' functions...

These will allow for operations like XOR, or for 'Transparency'... when a pixel is color 0

Logical commands are significantly slower, so should only be used when needed
We're going to need to check TR (Transmit Ready - ok to send/read data) and CE (Command Executing - vdp busy)

To do this we need to Select the status register number with 'normal' Register R15... then we read from the status port &99 to check the status of the register.

VDP Commands
The 9938 had a wide range of commands, you can see them below, we'll take a look at all of them!... NOTE: many commands work with bytes - and X co-ordinates must be even to work correctly.

Purpose Fill Vram From CPU Fast Copy (Y axis) Fast Copy Fill Logical Fill Vram From CPU Logical transfer data to CPU
Logical Vram to Vram Logical Fill Line Search (Xaxis) Pset Read Point Stop
R32 SX-L Src X



R33 SX-H Src X



R34 SY-L Src Y


R35 SY-H Src Y


R36 DX-L Dest X X X X X X

R37 DX-H Dest X X X X X X

R38 DY-L Dest Y X X X X X

R39 DY-H Dest Y X X X X X

R40 NX-L Num X X

R41 NX-H Num X X

R42 NY-L Num Y X X X X X X X X X

R43 NY-H Num Y X X X X X X X X X

R44 CLR Color X (first pixel)

X (Color) X (Color)

X (Color) X (Color) X (Color) X (Color)

R45 ARG Argument X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Dest) X (Dest)
R46 CMD/ LOP Command/
Logical Op
X (&F0) X (&E0) X (&D0) X (&C0) X (&Bx) X (&A0) X (&9x) X (&80) X (&70) X (&60) &(5x) &(40) &00

Readable Registers

S7= Pixel Data

S8/9 = X


HMMC (High speed move CPU to VRAM) - Copy data from CPU to VRAM
In this example, we'll transfer data from the Z80 (a sprite in this case) to VRAM

This will probably be the best way to get sprites from our disk/cartridge to the CPU

We need to transfer the first pixel with the command... then send the rest to the VDP data port &99.

We'll select an (X,Y) position, a Width & Height to define an area onscreen the data will go into.

In theory, We should wait for the TR Flag, but in practice, we don't need to!
We need to send data to registers 36-46... we're going to write 11 registers...

We push all the values for these registers on the stack, then use OUTI commands to send the data to the VDP

We need to set the indirect register to the first register we want to change (36) before we start our OUTIs... we do this by writing 36 to the control port then 128+17.... the '128+' tells the hardware to set the register number 17... the indirect register will autoinc, so we'll write to 36,37, 38 and onwards

This will start the command...

Next we need to select register 44... but we want to stop AutoINC this time... so we add 128.
Our sprite will be transferred from RAM to the screen

YMMM (High speed move VRAM to VRAM, Y coordinate only) - Fast Copy (Same Y)
We're going to define a range to copy... Notice we define height and Ysource... but not Width or Xpos...

Xpos is the same as Xdest... and the width is to the edge of the screen... this of course means the direction we copy is quite important!

Because the function is more limited, it's slightly faster than the normal version.
For this command We need to set Registers 34+

Like before, we push all the registers onto the stack, then execute a series of OUTI's to send the data...

With this command the VDP will start and do all the work itself, so we don't need to do anything after the command starts... the Z80 can get on with other work...

But if we want to tell the VDP to start another job we'd have to check the CE flag to make sure the task has finished
The result can be seen here!
The part of our code which does the OUTI from reg 34 (VDP_YMMM) doesn't have anything specific to the YMMM command
There will be other commands that also write from Reg34, but we'll keep separate versions to keep it clear in this example..

If you were using the code in your own program, some optimization would be a good idea!

HMMM (High speed move VRAM to VRAM) - Fast Copy
We're going to copy a sprite from VRAM to a VRAM...

During our game this will do most of our sprite copying as it's fast... however it will delete any background areas - so doesn't allow transparency (See LMMM)
Were going to need to fill all the registers from 32-46...

We're going to use the shadow registers for DestX/Y as we don't have enough registers for the regular parameters
The sprite will be copied to a second position onscreen.

HMMV (High speed move VDP to VRAM) - Solid Fill
The tile is misleading, but this command effectively allows us to fill areas with a single byte... this is the fastest way to clear the screen (CLS)

We specify an XY pos for the fill, a width and height and a color...
The HMMV command needs to fill registers 36+... Like most commands it does not need any action after the fill starts.
The requested areas will be filled.

If the nibbles of the color bit do not match (in this example &CD) the filled area will have vertical lines...

It would be nice to have a 'checkerboard' effect... where the nibbles swapped every other line, but the VDP cannot do this, so you probably want to just use solid colors (like &CC or &DD)

Lesson P49 - All MSX2 Bitmap Commands - Part 2/2
We looked at many fast commands last time... this time we'll look at the 'Logical' commands and a few other strange commands!

While slower, Logical commands allow for transparency!


LMMC (Logical move CPU to VRAM) - Copy data from CPU to VRAM
Like HMMC, LMMC transfers data from Z80 ram to the screen,

but this 'Logical' Version allows an operation to be passed in the C register...

we're going to test with an XOR (invert background with pixels) or NOT (Invert pixels)

Logical commands work in pixels, not bytes... so we need to pass our colors in the low nibble of bytes... this means two writes to the SendByteData port (&9B) for each byte in HL

Like HMMC we need to send the values to Reg36 onwards...

Once we've prepare the registers, we need to select Register 44 so the VDP can recieve the bit data
The two versions of the sprite will be shown...
The XOR one will be transparent (but background pixels will be inverted)
The NOT one will be a color negative.

Logical operations allow us to perform transparency... but this isn't really the best way of doing transparency for games... LMMM is faster as it's VRAM->Vram... we'll take a look at it soon!

LMCM (Logical move VRAM to CPU) - Read Pixels from VRAM to CPU
LMCM is the opposite of LMMC... it transfers data out of vram into the CPU...
BUT... because it's a Logical operation, it copy each pixel - not each byte...

We can specify an XY start pixel, a Width and a Height...

We can then read in color pixels from Status Register 7... in this example we'll store them in ram.
For this command We're going to need to populate registers 32+

We read in the colors from Status Register 7.
The Sprite was drawn to the screen, and read back to $D000

The data in $D000 will be the same as the sprite (originally at $8332) except the two nibbles of each byte will be split up... this is because we read back in PIXELS, not BYTES.

LMMM (Logical move VRAM to VRAM) - Logical Copy (good for transparent sprites)
We're going to try out every single Logical operation on our sprite, to see how the pixel data is effected.

We'll draw a strip of 20 pixels, and specify an operation with our predefined symbols... the operation will be specified in C

The LOG_Txx options use color 0 as a transparent color.
Pretty much like before, we need to fill registers R32 onwards...

This time however, we need to change the bottom nibble of the 'Command byte' as it's the 'Logical Operation'... we OR in the value in C to do this
The result can be seen below - along with the Logical operation and it's bits that are set in the low nibble of the Command byte (Reg 46)

LMMV (Logical move VDP to VRAM) - Logical Fill
Logical fill is slower than the high speed version, but it can be used for effects that may be useful.

In this example, we'll fill areas of the screen with the standard Implicit and XOR options.
Like the others, we need to set Registers 36 onwards
The results will be shown to the screen.

LINE - Draw a line
The VDP can draw lines for us...
The definition is slightly odd...
Usually NX is width - but for this command it's the length of the Long Side
Usually NY is height - but for this command it's the length of the Short Side

We specify a Direction as we often do, but in this case Bit 0 is the 'MAJ' bit... setting this flips the Long side.

We also specify the typical XY position, and a color for the line
We need to set Regsiters 36 onwards...

We also need to set the Color byte, apart from that the rest of the code is typical.
The Three lines will be shown to the screen... The middle one (light green) is flipped because of the MAJ bit.

SRCH - Search line for a color
This function may be useful for a graphics package, for example a fill routine.

It searches the current X line - either Left or Right - for a color... we can find the color itself, or the first pixel that isn't the color.

When we want to search, we fill registers 32 onwards as usual.

When the command completes however, we need to do some special things!

First we need to check the CE flag, to see the command really finished..

Next we check the BD flag (bit 4) of Status Reg 2 ... if it's 1 we found the pixel... if it's zero it was never found.

If we wound the pixel, we need to read in the low byte of the Xpos fro Status Reg 8... and the high byte from Status Reg 9... note unused bits (9+) are set to 1
In this example we found a pixel at 120 (we placed it there!)

PSET - Set a pixel
We can use PSET to set pixels - we can even use Logical operations

In this example, we XOR one of the pixels - effectively making it flash every other draw
We transfer our data to Register 36 onwards
The pixels will be drawn to the screen.

POINT - Read a pixel
We can read a point from the screen, and return the color of that pixel in A

We're going to define a function that will return the color for us.
The basics are the same as any other of our commands which use registers 32+

Most of the registers actually aren't needed (NX/Y DX/Y)... but it's easier just to give them dummy data.

Once we've run the command, we  read from Status Register 7... it will contain the color of the pixel
Our colors will be read from this screen... in this example one pixel was color 15, the other color 2 (from the PSET example)

STOP - Abandon ongoing Task
The VDP can only do one job at a time, but we can stop it and change the job!

In this test, we'll set the VDP to clear the screen white... wait a bit, then abort the job!
To abort the job, we just send a 0 to the command register R46
The Fill command only managed a few lines before we aborted it!

Phew! We've looked at all the commands!... The VDP is pretty powerful, and gives up lots of functionality...

Remember it's a second processor... the Z80 can do other things while it's drawing, but we'll have to check the CE flag (and wait if needed) before issuing another command!

Purpose Fill Vram From CPU Fast Copy (Y axis) Fast Copy Fill Logical Fill Vram From CPU Logical transfer data to CPU
Logical Vram to Vram Logical Fill Line Search (Xaxis) Pset Read Point Stop
R32 SX-L Src X



R33 SX-H Src X



R34 SY-L Src Y


R35 SY-H Src Y


R36 DX-L Dest X X X X X X

R37 DX-H Dest X X X X X X

R38 DY-L Dest Y X X X X X

R39 DY-H Dest Y X X X X X

R40 NX-L Num X X

R41 NX-H Num X X

R42 NY-L Num Y X X X X X X X X X

R43 NY-H Num Y X X X X X X X X X

R44 CLR Color X (first pixel)

X (Color) X (Color)

X (Color) X (Color) X (Color) X (Color)

R45 ARG Argument X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Direction) X (Dest) X (Dest)
R46 CMD/ LOP Command/
Logical Op
X (&F0) X (&E0) X (&D0) X (&C0) X (&Bx) X (&A0) X (&9x) X (&80) X (&70) X (&60) &(5x) &(40) &00

Readable Registers

S7= Pixel Data

S8/9 = X


Back to Page 4  | Continue to page 6


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ARM Downloads
ARM Cheatsheet
DevTools kit
ARM Platforms
Gameboy Advance
Nintendo DS
Risc Os

Risc-V Content
Learn Risc-V Assembly
Risc-V Downloads
Risc-V Cheatsheet
DevTools kit

MIPS Content
Learn Risc-V Assembly
MIPS Downloads
MIPS Cheatsheet
DevTools kit
N64 Platforms

PDP-11 Content
Learn PDP-11 Assembly
PDP-11 Downloads
PDP-11 Cheatsheet
DevTools kit

TMS9900 Content
Learn TMS9900 Assembly
TMS9900 Downloads
TMS9900 Cheatsheet
DevTools kit
TMS9900 Platforms
Ti 99

6809 Content
Learn 6809 Assembly
Learn 6309 Assembly
6809/6309 Cheatsheet
DevTools kit
6809 Platforms
Dragon 32/Tandy Coco
Fujitsu FM7
TRS-80 Coco 3

65816 Content
Learn 65816 Assembly
65816 Cheatsheet
DevTools kit
65816 Platforms

eZ80 Content
Learn eZ80 Assembly
eZ80 Downloads
eZ80 Cheatsheet
DevTools kit
eZ80 Platforms
Ti84 PCE

IBM370 Content
Learn IBM370 Assembly
IBM370 Downloads
IBM370 Cheatsheet
DevTools kit

Super-H Content
Learn SH2 Assembly
SH2 Downloads
SH2 Cheatsheet
DevTools kit
SH2 Platforms

PowerPC Content
Learn PowerPC Assembly
PowerPC Downloads
PowerPC Cheatsheet
DevTools kit
PowerPC Platforms

Work in Progress

Misc bits
Ruby programming

Buy my Assembly programming book
on Amazon in Print or Kindle!

Buy my Assembly programming book

Available worldwide!
Search 'ChibiAkumas' on
your local Amazon website!
Click here for more info!

Buy my Assembly programming book
on Amazon in Print or Kindle!

Buy my Assembly programming book

Available worldwide!
Search 'ChibiAkumas' on
your local Amazon website!
Click here for more info!

Buy my Assembly programming book
on Amazon in Print or Kindle!

Buy my Assembly programming book

Available worldwide!
Search 'ChibiAkumas' on
your local Amazon website!
Click here for more info!