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
P60
- Tape loading on the Amstrad CPC (5K subs special)
The 5000 sub special demo used a cassette file, and streamed data
in a couple of formats to the screen.
Lets take a look at the loading methods used.
CPC_TapeRLE.asm
5K_Demo.cdt
Making a tape with 2CDT
We're using 2CDT to build a tape file... Our test program has
two types:
2 files are saved normally with blocks (The RLE compressed Cinema)
1 file is saved headerless (The final thanks message)
2CDT takes a variety of switches we'll need:
;-n = start New tape
;-s 1 = Speed 2000 Baud (fast)
;-m 0 = Add Block data (use with &BC77)
;-t 1 = Turbo Loading blocks (default)
;-m 1 = Add Headerless (use with &BCA1)
Tape loading... Getting Started
Our demo is started with CALL &4000 ...
We're running on an AMSTRAD 6128 - this system starts with reading
from disk - we need to use the basic command "|TAPE"
There's no defined firmware command to do this, but we can do a 'FAR
CALL' (Call to ROM) with RST3
This command takes a pointer to 3 bytes... the first two are a
destination address (&CCFD for |TAPE) and the third is a ROM
NUMBER (7 for the DISK ROM which contains the command)
By default the CPC will show tape messages - we'll want to turn this
off.
We do this with &BC6B (CAS NOISY)... passing this a 0 in A
will enable messages (Default)... we want to turn them off so we set
A=1
Tape loading... Reading Bytes (With Blocks)
We're going to open a file on the tape... it's RLE data, and we'll
process it a byte at a time.
We need to specify a memory address of a 2k buffer - we do this with
DE
If we want to specify the filename to load, we specify a pointer to
the name in HL - and the filename length in B... alternatively we
can specify B=0 to just load the next file on the tape.
We then use function &BC77 (CAS IN OPEN) to open the cassette
file
we use &BC80 (CAS IN CHAR) to read in a single byte from the
file...
We're using the RLE
decoder we looked at previously to show the cinema graphic
Each section has a 'header byte'
If the whole byte is zero - then we're reached the end of the
compressed sequence
If the top bit is 1, the next sequence is RLE compressed... the
remaining bits are a bytecount (1-127)
If the top bit is 0, the next sequence is LINEAR uncompressed
data... the remaining bits are a bytecount (1-127)
The RLE decompressor has been modified to skip zero bytes, allowing
multiple layers to be overlaid to build up an image.
Layer 1
Layer 2
Combined
Result
Once we've loaded in the file we use &BC7A (CAS IN CLOSE) to
close the file.
If you don't
want to do anything clever with the file byte by byte, and want to
read in blocks, use &BC83
- CAS IN DIRECT
Specify a destination address with HL,and the file will be loaded
into ram.
Tape loading... Reading to ram (Headerless)
If we want to quickly load a large block of data, Headerless data
is fastest... we load headerless data with &BCA1 (CAS READ)
As there's no header, we don't specify a filename... there's no
blocks, so the data is quicker.
We need to specify a destination in HL, a size in DE... and a Header
sync byte in A (this needs to match the one used to build the file -
2CDT and the amstrad BIOS default to &16 for data)
This can be used for reading in a loading screen in one go.
Lesson
P61 - 2x and 4x sprite scaling with Lookup Tables on the Amstrad
CPC
Lets take a look at sprite scaling! in this example we'll use lookup
tables to effect Double and Quad size sprites on the Amstrad CPC.
We'll use a lookup table, so the scaled sprite will be about as fast
as a regular one.
CPC_Scale2x4x_Sprite.asm
This
example scales images on the CPC... but the principal will work on
any system which uses a bitmap screen (Like the SAM or Speccy)
Scaling sprites can be used for a neat effect - it could also be
used to save memory if you want huge graphics - of course a sprite
scaled 2x in both directions uses 1/4 of the size of an unscaled
sprite.
Sprite scaling and the Lookup tables
We're going to scale our Chibiko sprite - here it's shown at 1x ,
2x and 4x size.
The sprite file is the same as the one used in the simple series.
and the code used to display the 1x version is almost the same (It
uses different registers for convenience to match the 2x and 4x
versions)
We need some lookup tables to convert the bytes to 2x and 4x...
these contain the new scaled 'screen bytes'
There are 256 possible screen bytes at 1x
2x needs a 512 byte Lookup Table
4x needs a 1024 byte Lookup Table
As MODE 0 only has 2 pixels per byte, we don't actually need the 4x
lookup table in Mode 0
Building the Lookup Tables
Before we use the draw routines, we need to fill the LUT's...
The lookup tables are byte aligned
If the 2x LUT is at &16xx...
&16xx will be the First byte of the conversion for screen byte
xx
&17xx will be the Second byte of the conversion for screen byte
xx
The same is true for the 4x LUT... but it will use addresses
&18xx &19xx &20xx and &21xx for the 4 output bytes
We need to mask each pixel of the source byte, and fill the
destination byte with OR commands
We need to understand how the screen pixels work to do this... here
we're building the 2x scale for Mode 1
It's pretty much the same for the MODE 1 4x scale routine.
The MODE 0 2x scaling routine is almost the same, just the mask
and shifts are different.
There is no 4x LUT for Mode 0 - because there are only two pixels
per byte, we can use the 2x LUT for the 4x version (we just use each
entry twice)
Drawing the sprites!
We're going to draw our sprite!...
We have 3 different routines for each of the scales.
We use 'GetScreenPos' to calculate the screen ram location... this
is the same as in the Simple Series.
Here's the 1x routine, we read each pixel from the source bitmap
in HL... and transfer them to the screen at DE
When we want to scale the source sprite 2x we use the lookup
table.
We read a byte as before from HL... but this time we load the lookup
table into BC.
We swap C for the byte from HL - BC now points to the first
conversion for that byte
We transfer the first byte from BC to the screen (This is the first
of the 2x scaled equivalents of the byte from HL)
We increment B - BC now points to the second conversion for that
byte
We transfer the first byte from BC to the screen (This is the first
of the 2x scaled equivalents of the byte from HL)
We repeat the procedure for the next source byte.
The 4x procedure is the same... of course we use the 4x scaling LUT
form Mode 1this time.
This time We transfer 4 bytes from the LUT to the screen
We INC B four times in MODE 2 for the 4 destination byte.
In Mode 0 we use the 2x LUT - and we write each byte from the LUT
twice... INCing B twice - this is because there's only 2 pixels per
byte, so there's no need for another LUT
We can also use the 4x LUT for 8X if we want super blocky sprites
for some reason!
Lesson
P62 - AMX Mouse reading on the Amstrad CPC
The AMX mouse is an 8 bit mouse supporting both the Amstrad CPC and
the ZX Spectrum - it's a little unusual, rather than connecting via
a dedicated mouse port, it in fact 'emulates' a joystick!...
When the mouse is moved up, the 'UP' button is pressed... the same
for Down, Left and Right. and 'Fire' buttons
We could just play our regular game with the mouse, but really we
need to read it during an interrupt handler for smooth movement...
lets learn how!
CPC_Mouse.asm
Our Test program.
We're going to create a test program.
It will read the mouse via the joystick port and move a cursor
around the screen.
When you click the left mouse button, it will draw a 'Chibiko' (our
vampire mascot)
When you drag the left mouse button it will draw a blob trail.
For speed and simplicity, All sprites will be drawn with 'XOR'
meaning a sprite will be removed if it's drawn in the same position
twice.
Reading the Mouse (Joystick)
We read the joystick in the same way as the keyboard - we need to
select the row of the keyboard we want to read, and read that line
in via the AY
We read in line &49 - the line for Joystick 1 - this contains
all the joystick buttons the mouse use.
Keyboard Matrix
The Keyboard matrix uses 7 bits of line &49 for the Joystick - 4
directions and up to 3 firebuttons.
Fire 3 is undocumented, but is supported on the classic CPC as middle
mouse - however it is NOT supported on the CPC+
0
1
2
3
4
5
6
7
&40
C-U
C-R
C-D
F9
F6
F3
F-ENT
F.
&41
C-L
CPY
F7
F8
F5
F1
F2
F0
&42
c
[
RET
]
4
SHIFT
\
c
&43
^
-
@
P
;
:
/
.
&44
0
9
O
I
L
K
M
,
&45
8
7
U
Y
H
J
N
&46
6 J2-U
5 J2-D
R J2-L
T J2-R
G J2-F1
F J2-F2
B J2-F3
V
&47
4
3
E
W
S
D
C
X
&48
1
2
ESC
Q
TAB
A
CAPS
Z
&49
J1-U
J1-D
J1-L
J1-R
J1-F1
J1-F2
J1-F3
DEL
Our Mouse Routine
We're going to use a custom interrupt handler, this ensures
mouse movements wont be missed if our program gets slow.
We're going to need some variables
We'll store the mouse position - we keep the last position so we
know where the mouse was last time we drew it (so we can remove
the old sprite)
We use 16 bit X,Y co-ordinates so we can have 'half movements' -
we use the top byte of the X,Y pos for drawing...
Our drawing routine works in screen bytes - so the Mode 1 screen
is 80x200 - meaning we need to move 1 unit in the X axis for every
4 or so in the Y axis.
We need to process each of the 4 directions... Up, Down, Left
and Right
We check for each direction, and if it's pressed - we check the
current location
If we're already at the edge of the screen, we don't want to move
again, but if our current position is OK, we add or subtract the
move as required and update the cursor position.
We also test the fire button, and if it's down we set our fire
markers...
if it's released we clear the held marker. (The fire down marker
should be cleared by the routine that uses the mouse)
This example
just processes the left mouse button - we could mimic this for
the right and even center buttons.
Just remember - Fire 3 (Center) will not work on the CPC+!
Using the Mouse Routine
We're going to use our mouse... First we test if fire was just
pressed... if it was we'll show 'TestSprite2' (Chibiko) at our
current location
We need to clear the Fire down event to mark it processed
We're going to update the cursor - first we check if the old
position and new position are the same - if so we have nothing to
do.
If the cursor has moved, first we redraw the cursor in the old
position - as we're using XOR sprites this removes the old cursor,
Next we draw the cursor in the new position
Finally we copy the new cursor position into the old cursor
position - for the next run of the main loop
Finally we'll process Fire held... this routine only runs if the
cursor has moved.
If fire is held, we show a blob at the current position
We do NOT clear the fire held flag - it's cleared by the interrupt
handler.
Lesson
P63 - Kempston Mouse reading on the ZX Spectrum + SpecNEXT
The Spectrum can also use a mouse... the Kempston mouse - this is
supported by the classic spectrum and spectrum next... lets put it
to good use!
ZXS_KempsonMouse.asm
ZXN_KempsonMouse.asm
Mouse Routine
Reading the Kempston mouse is super easy!
Ww just read 3 ports
Reading &FBDF We can get an 'X
position'
Reading &FFDF We can get an 'Y
position'
Reading &FADF gets the Left and right
mouse buttons in the format %000000LR
Here is some sample code to read the XY pos of the mouse in HL -
and the Buttons in A
Note X and Y position read from &FBDF
and &FFDF do not give positions on the screen - they are
relative movements positions.
X=0 does not mean far left, nor does 255 mean far right... all
we know is if the X pos goes down the mouse moved left... and if
the Y pos goes down the mouse moved down.
Our Mouse Routine
We're going to need some variables
We'll store the mouse position - we keep the last position so we
know where the mouse was last time we drew it (so we can remove
the old sprite)
We use 16 bit X,Y co-ordinates so we can have 'half movements' -
we use the top byte of the X,Y pos for drawing...
We also need two bytes to compare the last XY position to the
current one (Xmove and Ymove)
First we test the fire button with port &FADF, and
if it's down we set our fire markers...
if it's released we clear the held marker. (The fire down marker
should be cleared by the routine that uses the mouse)
We read in the current X and Y position from &FBDF
and &FFDF
We subtract these from the last read in value to get a movement
distance - we flip the Y axis to match our screen co-ordinates.
We use these Xmove and Ymove values (now in Reg D and E) to
alter the mouse position.
When we add or subtract the movement from each axis, we need to
check if this causes an overflow,
If we go below zero, then we've gone over the top or left edge
of the screen...
If we go over X>256-7 or Y>192-7 then we've hit the right
or bottom of the screen.
We've now converted the movement to a valid onscreen mouse
position
Using the Mouse Routine
We're going to use our mouse... First we test if fire was just
pressed... if it was we'll show 'TestSprite2' (Chibiko) at our
current location
We need to clear the Fire down event to mark it processed.
We're going to update the cursor - first we check if the old
position and new position are the same - if so we have nothing
to do.
If the cursor has moved, first we redraw the cursor in the old
position - as we're using XOR sprites this removes the old
cursor,
Next we draw the cursor in the new position
Finally we copy the new cursor position into the old cursor
position - for the next run of the main loop
Finally we'll process Fire held... this routine only runs if
the cursor has moved.
If fire is held, we show a blob at the current position
We do NOT clear the fire held flag - it's cleared by the mouse
reading routine.
Lesson
P64 - Mouse reading on the MSX
The MSX aslo has a mouse - it connects to the Joystick port, but
is not read in the same way as a normal joystick.
We have to set some bits of the port, and we get a pair of
nibbles for each X and Y axis - these are an 'offset' which we
can use to build a 'position'... lets learn how.
MSX_MouseText.asm
MSX_Mouse.asm
Mouse Routine
Like the Joystick The mouse is read through
AY ports A and B...
Unlike the Joystick, we need to send data
on pin 8 to select the High/Low nibble of the axis.... our first read
will get the X axis offset, the second will get the Y axis.
The bits of the ports are shown below
Port A (&A2 - AY Reg 14)
Bit
Direction
Details
Joy
Function
Mouse
Function
0
IN
J1-Pin 1
/ J2-Pin 1
Up
Low/High
Bit 0
1
IN
J1-Pin 2
/ J2-Pin 2
Down
Low/High
Bit 1
2
IN
J1-Pin 3
/ J2-Pin 3
Left
Low/High
Bit 2
3
IN
J1-Pin 4
/ J2-Pin 4
Right
Low/High
Bit 3
4
IN
J1-Pin 6
/ J2-Pin 6
Fire 1
Right
Button
5
IN
J1-Pin 7
/ J2-Pin 7
Fire 2
Left
Button
6
IN
Key
Layout Select (JP version only)
7
IN
Cassette
Read
Port B (&A1
- AY Reg 15)
Bit
Direction
Details
Joy
Function
Mouse
Function
0
OUT
J1-Pin 6
Set to
1
Set to
1
1
OUT
J1-Pin 7
Set to
1
Set to
1
2
OUT
J2-Pin 6
Set to
1
Set to
1
3
OUT
J2-Pin 7
Set to
1
Set to
1
4
OUT
J1-Pin 8
Get
Low/High Nibble
5
OUT
J2-Pin 8
Get
Low/High Nibble
6
OUT
Port A
Input Select
Joy 1/2
Mouse
1/2
7
OUT
Kana Lamp
On/Off
Reading from the mouse
When we want to read a nibble from an axis, we need to send the
configuration to port B to set up the mouse,
We then read from port A to get the data from the mouse
We need to send the correct data to port B - the bits we need to
set are different depending if our mouse is in joystick port 1 or
2
The settings here are reading a mouse in joystick port 2 - use the
REMmed out versions for joystick port 1
Here we're reading and combining the high and low nibbles - this
gets one axis.
We run the GetMouseAxis command twice - this reads both the X
and Y axis...
A will also contain the Left/Right mouse buttons in bits 4,5 - we
set all the other bits to 1
Here's a simple routine to read in the mouse data, and convert
it to an XY position in BC... A contains the LR Mouse buttons.
Note X and Y position read from here do
not give positions on the screen - they are relative movements
positions compared to the last read - even when we add them to
previous reads, they are still only a 'relative movement - we
need to covert this to a screen co-ordinate.
Our Mouse Drawing Routine
We're going to write a little mouse drawing routine,
It will show a cursor that's limited to the screen area, we'll
be able to click to draw a 'Chibiko', and click and drag to
draw blobs.
This is written on the MSX2
We're going to need some variables
We'll store the mouse position - we keep the last position so
we know where the mouse was last time we drew it (so we can
remove the old sprite)
We use 16 bit X,Y co-ordinates so we can have 'half movements'
- we use the top byte of the X,Y pos for drawing...
First we test the fire button - it's still in the
Accumulator, and if it's down we set our fire markers...
if it's released we clear the held marker. (The fire down
marker should be cleared by the routine that uses the mouse)
We transfer the read in XY pos into DE NOTE: The mouse reading routine has been flipped... a NEG
command has been added to GetMouseNibble
When we add or subtract the movement from each axis, we need
to check if this causes an overflow,
If we go below zero, then we've gone over the top or left edge
of the screen...
If we go over X>256-7 or Y>192-7 then we've hit the
right or bottom of the screen.
We've now converted the movement to a valid onscreen mouse
position
This
code uses MSX2 VDP commands to do the graphics work... if
you don't know about them, you can learn more Here
Using the Mouse Routine
We're going to use our mouse... First we test if fire was
just pressed... if it was we'll show 'TestSprite2' (Chibiko)
at our current location
We need to clear the Fire down event to mark it processed.
We're going to update the cursor - first we check if the old
position and new position are the same - if so we have nothing
to do.
If the cursor has moved, first we redraw the cursor in the old
position - as we're using XOR sprites this removes the old
cursor,
Next we draw the cursor in the new position
Finally we copy the new cursor position into the old cursor
position - for the next run of the main loop
Finally we'll process Fire held... this routine only runs if
the cursor has moved.
If fire is held, we show a blob at the current position
We do NOT clear the fire held flag - it's cleared by the mouse
reading routine.
Lesson
P65 - Mouse reading on the Sam Coupe
Lets take a look at the Sam Coupe mouse.
It can be read from the keyboard matrix, but returns 12 bit
values, lets learn how it works!
SAM_MouseCursor.asm
SAM_Mouse.asm
Generally,
the tech manual"sam-coupe_tech-man_v3-0.pdf" is pretty awesome,
but the documentation on the mouse is a massive pile of suck!...
Either that or the author of these tutorials is too stupid to
understand them (Either is possible!)
As the documentation was lacking, today's example was written by
trial and error, it seems to work fine, but no promises can be
made!
Mouse Port on the SAM
The SAM Coupe mouse is read
from port &FFFE,
We need to do sequential reads from that port, and look at the
bottom 4 bits.
We need to combine combine 3 nibbles to build the 12 bit X and Y
movement.
Successive reads from &FFFE will return the following data:
Port
&FFFE
Bit
Read
Number
7
6
5
4
3
2
1
0
1
X
X
X
X
-
-
-
-
2
X
X
X
X
-
-
-
-
3
X
X
X
X
-
R
C
L
4
X
X
X
X
Y-High Nibble
5
X
X
X
X
Y-Mid Nibble
6
X
X
X
X
Y-Low Nibble
7
X
X
X
X
X-High Nibble
8
X
X
X
X
X-Mid Nibble
9
X
X
X
X
X-Low Nibble
We need to read in 3 times from &FFFE to start, the last
gets us the mouse buttons,
Now we do 3 reads, these will be the Y axis
Then we do 3 more reads, these will be the X axis
Our Mouse Routine
We're going to write a mouse reader which will draw a cursor
onscreen.
Clicking the left button will draw a 'Chibiko' mascot, dragging
will draw a trail of blobs,
The sprites are XOR, so drawing twice will remove a sprite.
We're going to need some variables
We'll store the mouse position - we keep the last position so we
know where the mouse was last time we drew it (so we can remove
the old sprite)
We use 16 bit X,Y co-ordinates so we can have 'half movements' -
we use the top byte of the X,Y pos for drawing...
First read in 3 times and test the fire button with port &FFFE,
and if it's down we set our fire markers...
if it's released we clear the held marker. (The fire down marker
should be cleared by the routine that uses the mouse)
We use the DoAxis function to read in the Y and X move
We use these Xmove and Ymove values (now in Reg D and E) to
alter the mouse position.
When we add or subtract the movement from each axis, we need to
check if this causes an overflow,
If we go below zero, then we've gone over the top or left edge
of the screen...
If we go over X>128-7 or Y>192-7 then we've hit the right
or bottom of the screen.
We've now converted the movement to a valid onscreen mouse
position
Using the Mouse Routine
We're going to use our mouse... First we test if fire was just
pressed... if it was we'll show 'TestSprite2' (Chibiko) at our
current location
We need to clear the Fire down event to mark it processed.
We're going to update the cursor - first we check if the old
position and new position are the same - if so we have nothing
to do.
If the cursor has moved, first we redraw the cursor in the old
position - as we're using XOR sprites this removes the old
cursor,
Next we draw the cursor in the new position
Finally we copy the new cursor position into the old cursor
position - for the next run of the main loop
Finally we'll process Fire held... this routine only runs if
the cursor has moved.
If fire is held, we show a blob at the current position
We do NOT clear the fire held flag - it's cleared by the mouse
reading routine.
Lesson
P66 - Quadtree video on the CPC (10k special) - Part 1/2
When my youtube channel hit 10,000 subs, I made a little demo
which streamed a pre-rendered video
Lets take a look at the video compressor I wrote!
CPC_QuadTrees10K.asm
What are Quadtrees!?
Quadtrees are a way of breaking up an image into large blocks,
then splitting those blocks into 4 smaller blocks when required.
This is intended to combine the advantages of large block sizes
(Small bytesize for simple images) and the advantage of small blocks
(flexibility for varied images)
Suppose we want to compress this image ->
First we split the image up into large blocks (in our example's
case 64x64)
Each of these blocks will be considered
Lets take a look at this block
This block is quite varied, so we probably want to split it into
four 32x32 blocks (Yellow).
The TopRight block is completely empty so we'll solid fill it.
The BottomLeft block is quite complex - maybe we should store it at
high resolution
The TopLeft and BottomRight blocks are also rather varied, so maybe
these should be split into four 16x16 blocks (blue), and considered
separately!
This repeated splitting into quads is known as a quadtree.
The compression techniques
Because we're working on a lowly 8 bit system we won't be using
Jpeg compression
To 'compress' the blocks, we'll simply store them at different
resolutions
Something detailed, like the castle, we'll store at high
resolution.
Something vague, like the mountain, we'll store at lower
resolution.
We'll use the following possible compression algorithms:
1. Store the block as as solid filled color (or transparent)
2. Store the block at 1x1 (full resolution)
3. Store the block at 1x2 (half vertical resolution - only works
for 8x8 blocks)
4. Store the block at 2x2 (half resolution - only works for 8x8
blocks)
5. Store the block at 4x4 (quarter resolution - only works for
16x16 blocks)
You'll notice most of our 'compression
algorithms' only work for 8x8 or 16x16 blocksize... we'll have
to split a block until we get to one of those sizes.
This may seem wasteful, but it also has advantages:
1. We can hardcode our program to work at a fixed size to gain
speed.
2. Small blocks are more likely to be repeated in other places,
so we can 'reusue' the bitmap data for two small blocks.
Don't be
surprised if the resulting images aren't so great... We're stuck
between some SUCK and a HARD place with this project!
As we're compressing hundereds of frames, We need to have speed
and small sizes, so we're going to have to make tough choices!
The 1x2 resolution is chosen because it's easy to do on the CPC,
and gives nearly as good quality as 1x1...
2x2 and 4x4 are done via lookup tables... we've covered that before.
Making the video
The video is split into 3 sections with 450 frames in total, and
uses palette switching for extra effect!
The video is stored on a 481k CPC+ cartridge, which allowed for near
instant bank switching, so no loading, and no 128k memory
requirement.
In theory the video could be run from 512k ram in the same way, but
the memory layout would need reconfiguring.
The Scene was rendered with the open source 'Blender' 3D rendering
program.
The video was exported as a series of PNG files.
A special function was added to Akusprite Editor to load a video
sequence.
The video can be exported with Save
Quadtree Video
The default quality setting can
be set in File format... this is used to choose the compression for
each block
The default can be overridden on a per frame basis with the Setting option
The Quadtree routine
When we want to process a
frame, we use the ProcessQuadTrees routine. This is a recursive
routine which is also used when a block is split into quads
BC is the topleft corner of the block.
DE is the width and height of the block (in 8 pixel blocks, so
&0808 is 64x64)
IX is the width and height in blocks... so &0403 is 04*64 x
03*64 = 256x192
the DoQuadTree routine will decide on the correct algorithm for
decoding the block,
This routine is executed for each block in the area.
When we want to split a block into 4, we have an alternate
version, to save a little time on loops.
To process a block, we read in a byte from the command datastream.
The top nibble defines the block type, the purpose of the bottom
nibble depends on the block type.
Solid filled block
When we want to solid fill a block we have two routines to handle
this
The color to fill the block is defined by the bottom nibble of the
block definition.
A 'general' one for any size, and a fast '8x8 pixel' version.
Color 15 is the transparent color, so if we're filling with that,
then this is a block to be ignored.
Otherwise we calculate the screen VRAM destination.
We use PUSH commands and stack misuse to fast fill.
We also do 8 consecutive lines 'unwrapped' to increase speed.
Working in 8 line blocks allows us to optimize our 'nextline'
routine, as 7 consecutive 'nextlines' within a block are simple, but
the 8th is more complex.
the 8x8 routine is even simpler, as we can just do 1 push (2 bytes
- 8 pixels) per line, and not worry about getnextline
Full Resolution blocks
Bitmap blocks use a pixel data stream (PixelBlock),
however so that repeated blocks are only stored once this is stored
separately to the command stream.
The address in the bitmap stream is stored either as 11 bit, or 19
bit
This means there is either one address byte, plus 3 bits from the
low nibble of the data block definition, or two bytes, plus 3 bits.
The bitmap data is aligned so The bottom two bits of the address are
always zero, so a 2 byte address supports a range of 512k (for bank
switching support)
The Fullres routine only works on 8x8 blocks, which
makes screen address adjustment simple and removes the need for
loops
We use stack misuse to quickly read data in from the source bitmap.
Half and Quarter Res
The Half and Quarter resolution compressors use a
lookup table to quickly convert a byte in the stream to 2 or 4
screen bytes.
2 bytes are read in from the lookup table for each source byte.
A source byte is loaded from the stream, and this is used as the low
byte in the lookup table, the High byte is incremented to get
the second destination byte.
Here we've
looked at the concept of Quadtrees and how the blocks are decoded
and displayed.
Next time we'll take a look at the CPC+ Cartridge, and some of the
tricks we've had to do to get things working!
Lesson
P67 - Quadtree video on the CPC (10k special) - Part 2/2
When my youtube channel hit 10,000 subs, I made a little demo which
streamed a pre-rendered video
Lets take a look at the CPC+ code and cartridge routines.
CPC_QuadTrees10K.asm
Building our cartridge
We first compile our sourcecode to a binary file and then convert
this to a CPR using BuildCPR.
Were including a modified OS,Basic and Amsdos rom which simulate a
disk system using the cartridge. - Though our program doesn't
technically need disk or basic support!
We're using BUILDCPR to covert these to a CPR
OS.ROM BASIC.ROM and AMSDOS.ROM were taken from the NoCart
program, however BuildCPR gives us more flexibility.
However we have modified the ROM source of patchdos.asm
with a new initialization routine.
Our 'bootstrap' is at memory address &dc00, however it transfers
itself to address &A100 so that the rom can be paged out.
It pages in 3 banks of the cartridge, and copies &A000 bytes of
the ROM to &0100 in ram, then executes address &0100.
Our program needs to run from this address and take control!
Our program starts at address &100
Thanks to the bootstrap it will be running in ram!
The Framelist
AkuSprite Exports the bitmap data for the frames and the Command
Stream, However it also exports a 'template' of the image list with
pointers to the addresses of the frame data.
This is intended to allow frames to be held onscreen longer, or
repeated to save memory, however neither function was actually used.
BUT... We did inject some 'CPC+ palette switch commands' into the
stream!
The Frame processing sequence goes through each frame in the
Sequence.
First it checks if the frame is a palette switch (Speed=255) if it
is, the address of the palette is loaded in and switched via the
CPC+ functions.
Next it's checked if both bytes of the address are &FF &FF -
this marks an end of stream (a real frame would never start at the
end of a block!)
Anything else is a legit frame, so the bank is paged in and the
frame is shown.
Once a frame has been shown we test the delay, and see if drawing
the frame has taken longer than the delay, if not we wait a while
until enough time has passed - this is to control the speed, as some
frames are very slow, but others are super fast.
Rom Paging
There are 3 parts to our video.
1. The Framelist - This is used for the video sequence... it's
always in memory.
2. The Command Stream - This defines the blocks that make up each
data... It's paged in as needed
3. The Pixel Block data - This is used for bitmap blocks, it's
separate from the commands so multiple blocks can share the same
bitmap data... It's paged in as needed
We need to ensure 2 and 3 start in a new 16k bank... BUT we also
need to offset these by 256 bytes, to compensate for the fact our
program starts from address &0100, but appears in rom at the
start of a bank &0000
The Frame routine includes a 1 byte 'bank number' so the correct
bank can be paged in to process the Command Stream
If a frame spans over two banks, it can request a bankswitch to move
to the next bank
The Two byte addresses for Pixel blocks include a 5 bit bank
number (3 low bits in the low nibble + top 2 bits in the two byte
address
CPC+ Palette
The QuadTree video decoder
doesn't really use any CPC+ features (except rom switching) - so can
be used on a classic CPC
However we use the CPC+'s more flexible palette to allow for nicer
colors.
ChibiTracks for chibi beats!
The music of the demo is basically the same theme as the original
ChibiAkumas game, however it has been converted to my new
ChibiTracks music player!
ChibiTracks is a multiplatform player, which uses the ChibiSoundPro
platform specific driver.
We play music during the interrupt handler.
ChibiTracks and it's requirements are included as ASM.
The interrupts on the CPC occur at 300hz... but our music needs to
play at 50hz, so we skip 5/6 interrupts.
Chibitracks only uses AF,BC,DE,HL and IX - it does not use IY or
shadow registers!
ChibiSound
PRO and ChibiTracks support all the Z80,6502 and 68000 systems
covered in these tutorials.
But We'll take a look at it in future lessons!
Lesson
P68 - RLE+Bitplanes for lossless compression on the CPC
During my experiments with video, I also made a test of image
compression. RLE compression works best with long strings of
repeated bytes, and to try to increase their occurrence I split the
image into 2 bitplanes. It's slow to draw, but saved a lot of
memory!
CPC_BitplaneImage_Overscan.asm
Our bitplane data
We're going to compress an 'ovescan image' of 380x272.
The raw image is 27,278 bytes... if we RLE compress this it would go
down to 13,425
BUT... if we split it into two bitplanes and RLE compress, the image
will be a total of 11,144... saving 2,281 bytes for the identical
resulting picture!
We're going to split the image into 2 bitplanes, and also as
horizontal bytes are very likely to be different, but vertical ones
may be more similar, we'll work in 8x8 blocks.
Coincidentally this is the same file format as the Gameboy uses!
The file needs to be RLE compressed, and split into two.
Akusprite Editor has an option to do this!
We'll include the two files in our source.
Our RLE compressor
We're using a flexible decompressor.
It uses IY as a pointer to the source file.
IXL is the bytecount for RLE data (all bytes same)
IXH is the bytecount for Linear data (all bytes different)
BC,DE,HL and shadow regs are unused.
call RLEDecompress to Start processing stream IY
call RLEGetNextByte to Move to the next byte - read from (IY) to get
the result
Setting up our screen
We're going to set up the screen, we need to set up all the CRTC
registers - you can see the full list here
We use port &BCxx to select register xx
We use port &BDxx to set the selected register to value xx
&0C/D define the base screen address, and also allow the
32k screen we need for overscan.
Here are the values we're using! Our screen is huge, and spans two
16k banks...
Bank 1 is &8200-&BFFF
Bank 2 is &C000-&FFFF
Memory <&8200 is free, so we'll use it for our stack!
Memory &0000-&3FFF is free so we can use IM1,
Memory &4000-&7FFF is free so we can use ram bank switching
but there's a slight memory 'gap' between them we'll have to deal
with!
We're going direct to the hardware to set our colors.
We use port &7Fxx to do this.
We need to use hardware color numbers
Drawing our image
We XOR in our image data, so we clear the screen first
We point IY to our source data, and execute RLEDecompress to
initialize the RLE data stream.
HL points to the VRAM destination
We work in 8x8 tiles.
First we go across the screen - 48 tiles total (384 pixels)
Then we go down the screen - 34 strips total (272 lines)
We'll do bitplane 0 - The top 4 bits of a byte are bitplane 0 in
mode 1
We read a byte from the RLE stream (IY)
We take the top 4 pixels, mask bitplane 0 and write them to the
screen... we move 1 byte to the right
We take the bottom 4 pixels, mask bitplane 0 and write them to the
screen... we move back left
we then move down 1 line (by adding &0800 to HL) and repeat.
RLEGetNextByte moves IY to the next byte
We move across the screen 1 tile (2 bytes) and repeat until we've
done an entire horozontal strip of tiles.
We need to move down 1 strip, and when we get to the end of the
&8000-&BFFF bank, we manually step into the &C000 bank
We repeat until bitplane 0 is entirely drawn
We now do the same for bitplane 1 - The bottom 4 bits
of a byte are bitplane 0 in mode 1
This example was
just an experiment in saving a little memory by re-organizing
data.
Take a look at the 10k subs example for more impressive 'lossy'
compression - of course the video in that example is basically a
sequence of images.
Lesson
P69 - ChibiSound PRO on the CPC, MSX and Spectrum (128 - AY)
Lets take another look at sound!... we'll make an improved
multiplaform sound driver for music!
CPC_V1_ChibiSoundPro.asm
ChibiSound PRO!
ChibiSound is the sound driver that handles the particularities of a
system, there is typically one driver per system, though the CPC and MSX
drivers are essentially identical except for the AY register setting
routines.
The original 'ChibiSound' gave us one channel, one
Volume bit, six pitch bits, and the ability to turn noise on. Pitches were
not matched across systems, so sound 32 won't sound the same on all
systems.
The updated 'ChibiSound Pro' gives us all the channels
provided by the hardware, 8 volume bits, 16 pitch bits, and the ability to
turn noise on. Pitches were not matched across systems, however the
'ChibiOctave' lookup table provides values which ARE matched across all
systems.
ChibiSound PRO is essentially a reduced subset of AY functionality, and
was designed on the Z80 - it's 'PRO' suffix is a parody of the
'SoundBlaster PRO' - which could only do 8 bit sound so wasn't up to
professional standards! (neither is ChibiSound PRO)
ChibiSound PRO provides a standard interface to the underlying hardware,
it allows the following features to be set for each channel on the
underlying hardware:
Function
Register
Notes:
Channel Number (bit 0-6)
Noise On/Off (bit 7)
L
Multiple channels can be supported, but on single channel systems
only Channel 0 will be sure to play.
If possible Channel 0 will be a center channel, Channels 1+ may be
left/right
Noise bit turns the noise effect on (1) or off (0) - this can be set
on any channel, if the underlying hardware only supports one noise
channel, this will be resolved by the driver.
Volume
H
Set volume of the channel (0-255). Higher numbers are louder. O is
off
Pitch
DE
Set the pitch of the channel (0-65535). Higher numbers are higher
pitch.
Using DE does not standardize the resulting pitch - however a
'Lookup table' of notes 'ChibiOctave' provides a standardized way of
getting the correct DE value to get a pitch correct note on the
platform.
Chibisound PRO does not offer features like Envelope, LFE etc, as providing
consistent functionality across different platforms would not be realistic.
The
new driver is a big improvement on the old one but doesn't really
deserve the PRO suffix!
It's a parody of the early 'Soundblaster Pro' sound cards, which
could only do 8 bit digital sound, so weren't really of 'pro spec'
either!
AY Sound Chip
The AY uses a series of 8 bit registers to control the 3 sound channels.
To set a register, we must first select the register number by writing a
byte to $FF8800, then send the byte of
data to $FF8802