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

	Model A	Model B	Master
Cpu	2mhz 6502	2mhz 6502	2mhz 6502
Ram	16k	32k	128k
Vram	Uses internal memory
Resolution	640×256	640×256	640×256
Colors	8	8	8
Sound chip	SN76489 (4 channel)	SN76489 (4 channel)	SN76489 (4 channel)

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

Video Registers
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

System Vectors

Routine		Vector
Name	Address	Name	Address	Summary of function
		UPTV	0222	User print routine
		EVNTV	0220	Event interrupt
		FSCV	021E	File system control entry
OSWRSC	FFB3	–	–	Write byte to screen
OSRDSC	FFB9	–	–	Read byte from screen
OSFIND	FFCE	FINDV	021C	Open or close a file
OSGBPB	FFD1	GBPBV	021A	Load or save a block of memory to file
OSBPUT	FFD4	BPUTV	0002	Save a single byte to file from A
OSBGET	FFD7	BGETV	0216	Load a single byte to A from file
OSARGS	FFDA	ARGSV	0214	Load or save data about a file
OSFILE	FFDD	FILEV	0212	Load or save a complete file
OSRDCH	FFE0	RDCHV	0210	Read character (from keyboard) to A (Interrupts must be on! - CLI)
OSASCI	FFE3	–	–	Write a character (to screen) from A plus LF if (A)=&0D
OSNEWL	FFE7	–	–	Write LF,CR (&0A,&0D) to screen
OSWRCH	FFEE	WRCHV	020E	Write character (to screen) from A
OSWORD	FFF1	WORDV	020C	Perform miscellaneous OS operation control block to pass parameters
OSBYTE	FFF4	BYTEV	020A	Perform miscellaneous OS operation using registers to pass parameters
OSCLI	FFF7	CLIV	0208	Interpret the command line given
		IRQ2V	0206	Unrecognised IRQ vector
		IRQ1V	0204	All IRQ vector
		BRKV	0202	Break vector
		USERV	0200	Reserved

Interrupt Vectors

The standard 6502 interrupt vectors from $FFFA+ are ROM, however these jump to vectors in low memory addresses. IRQ and BRK interrupts back up A at address $00FC and push X and Y onto the stack in that order.

From	To	Function	Registers Pushed
$0202	$0203	BRK	STA $FC TAX PHA TAY PHA
$0204	$0205	IRQ	STA $FC TAX PHA TAY PHA
$0D00	$0D01	NMI

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

7	6	5	4	3	2	1	0
K	R	R	R	C	C	C	C

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