6502 Assembly programming for the BBC Micro B
| The BBC Micro was made by Acorn for
the UK's public broadcasting system, it was presented as part of the
TV show "BBC Micro Live" The BBC had highly configurable hardware, with support for many external devices, and even non 6502 coprocessors... it's Basic even supported in-line assembly language! the Model A was the cheaper model, however it's 16k limit will be rather restrictive, so we will be covering the more usable 32k system the Micro B |
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ChibiAkumas Tutorials
Lesson P1
- Bitmap Functions on the BBC
Lesson
P10 - Joystick Reading on the BBC
Lesson
P17 - Palette definitions on the BBC
Lesson
P22 (z80) - Sound with the SN76489 on the BBC Micro
Write the regsiter you want to set to $FE00
then write the new value to $FE01
| RegNum | register description | Mode
1 320x256 4 color |
||
| $FE20 | Screen mode | $D8 | ||
| 0 | Horizontal total | $7F | ||
| 1 | Horizontal displayed characters | $50 | ||
| 2 | Horizontal sync position | $62 | ||
| 3 | Horizontal sync width/Vertical sync time | $28 | ||
| 4 | Vertical total | $26 | ||
| 5 | Vertical total adjust | $00 | ||
| 6 | Vertical displayed characters | $19 | ||
| 7 | Vertical sync position | $22 | ||
| 8 | Interlace/Display delay/Cursor delay | $01 | ||
| 9 | Scan lines per character | $30 | ||
| 10 | Cursor start line and blink type | $00 | ||
| 11 | Cursor end line | $08 | ||
| 12 | Screen start address H (Address /8) | $30 | ||
| 13 | Screen start address L (Address /8) | |||
| 14 | Cursor position H | |||
| 15 | Cursor position L | |||
| 16 | Light pen position | |||
| 17 | Light pen position | |||
| 18 | Cursor width (BBFW) |
Physical Colors
the way Screen Bytes map to visible colors is strangely configurable, they are defined by the "Video ULA palette"... which maps nibbles to colors... if we map colors wrong, the same byte may appear a different color depending if it's on odd or even vertical strips
| Color Num | EOR | Color |
| 0 | 7 | Black |
| 1 | 6 | Red |
| 2 | 5 | Green |
| 3 | 4 | Yellow |
| 4 | 3 | Blue |
| 5 | 2 | Magenta |
| 6 | 1 | Cyan |
| 7 | 0 | White |
Hardware Addresses
| From | To | Purpose | Details |
| $FE00 | $FE07 | 6845
CRTC
Video controller 18 |
Set these to change screen mode |
| $FE08 | $FE0F | 6850 ACIA Serial controller 20.3 | |
| $FE10 | $FE1F | Serial ULA Serial system chip 20.9 | |
| $FE20 | $FE2F | Video ULA Video system chip 19 | Set these to change screen mode |
| $FE30 | $FE3F | 74LS161 Paged ROM selector 21 | |
| $FE40 | $FE5F | 6522 VIA SYSTEM VIA 23 | Sound & Keyboard |
| $FE60 | $FE7F | 6522 VIA USER VIA 24 | |
| $FE80 | $FE9F | 8271 FDC Floppy disc controller 25.1 | |
| $FEA0 | $FEBF | 68B54 ADLC ECONET controller 25.2 | |
| $FEC0 | $FEDF | uPD7002 Analogue to digital converter 26 | |
| $FEE0 | $FEFF | Tube ULA Tube system interface 27 |
Sound Controller - SN76489
The Sound Chip shares a port with the keybord... Before we can send any data to the sound chip, we have to set the port to WRITE... we do this by writing 255 to address $FE43 (we only do this once)
We've covered the sound chip in the Z80 tutorials here
Once we've done that, we can write our data to $FE41 in the format below
| Bits | |||||||||
| Command | Bit Details | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Format Template | L=Latch C=Channel T=Type XXXX=Data | L | C | C | T | D | D | D | D |
| Tone - Command 1/2 | C=Channel L=tone Low data | 1 | C | C | 0 | L | L | L | L |
| Tone - Command 2/2 | H= High tone data (Higher numbers = lower tone) | 0 | - | H | H | H | H | H | H |
| Volume | C=Channel (0-2) V=Volume (15=silent 0=max) | 1 | C | C | 1 | V | V | V | V |
| Noise Channel | (Channel 3) M=Noise mode (1=white) R=Rate (3=use tone 2) | 1 | 1 | 1 | 0 | - | M | R | R |
Sheila ADC - Analog to Digital Converter (Joystick)
The Joystick is analog on the BBC... we need to read UD and LR, which will return a value from 0-255....
When it comes to reading the Fire, we use $FE40 - part of the sound/keyboard controller!
| Port | R/W | Purpose | Bits | Details | |
| $FEC0 | W | Data Latch / Conversation Start |
----MFCC | M=Mode (0=8 bit 1=10 bit)... F=Flag (usually 0)... CC=Channel (0/1 = joy1 2/3=joy2) | |
| $FEC0 | R | Status | CBMMm-CC | C=Conversation complete (1=no)...B=busy... M=top two bits of conversiation... m=mode (8/10 bit)... CC=Channel | |
| $FEC1 | R | High Data byte | DDDDDDDD | 8 Bit Data | |
| $FEC2 | R | Low Data byte | DDDD---- | extra 4 low bits of 10/12 bit data |
Key Reading
| Key reading on the BBC is a little
weird and rather poorly documented Essentially you have to select a row (0-8) and Column (0-9), then read in the state of each key one bit at a time!.. these are both read and written to port $FE4F
When Written Bits 0-3 (Marked C) select the Column (0-9) and bits 4-6 (Marked R) select the Row (0-8)... Bit 7 has no purpose When Read Bit 7 returns the Keystate and bits 6-0 have no purpose An example working piece of code is shown to the right, it's partially based on the disassemblies of the firmware. You will need a PrintHex command to show the read byte to screen. |
LDA #$7F
;set port A for input on bit 7 others outputs STA $FE43 LDA #$03 ;stop auto scan STA $FE40 ; This section may not be needed ;LDA #$0F ;select non-existent keyboard column F (0-9 only!) ;STA $FE4F ; ;LDA #$01 ;cancel keyboard interrupt ;STA $FE4D ; ldy #0 KeyNextLine: ldx #8 tya KeyNextBit: pha sta $FE4F lda $FE4F rol pla rol z_as clc adc #%00010000 dex bne KeyNextBit lda z_as iny sta (z_hl),y jsr PrintHex tya cmp #8 bne KeyNextLine LDA #$0B ;select auto scan of keyboard STA $FE40 ;tell VIA |
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