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 H1 - Hello World on the BBC Micro!
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
Lesson P37 - Screen settings with the CRTC on the BBC Micro!
Lesson P45 - Sound on the BBC (ChibiSound Pro)
Lesson S1 - Bitmap Drawing on the BBC
Lesson S10 - Joystick Reading on the BBC
Lesson S20 - Sprite Clipping on the BBC
Lesson Photon2 - BBC - ASM PSET and POINT for Pixel Plotting

Useful Resources
Advanced User Guide PDF - Official documentation on all the hardware
BBC-OS-1.2-Annotated Disassembly - Useful for seeing how firmware interrupt handlers work!

Memory Map

Memory Range	Purpose
$0000-$006F	BASIC language
$0070-$008F	free for user routines
$0090-$00AF	allocated to machine operating system
$00B0-$00CF	allocated to current filing system
$00D0-$00FB	allocated to machine operating system
$00FC	User IRQ routine save slot for register A
$00FD-$00FE	Address following detected BRK instruction
$00FF	The top bit is set during an ESCAPE condition
$0100-$01FF	6502 stack
$0200-$02FF	Operating system workspace and indirection vectors
$0300-$03FF	Miscellaneous workspace
$0400-$07FF	Language ROM workspace
$0800-$08FF	Miscellaneous workspace
$0900-$09FF	RS423 transmit, cassette, sound and speech workspace
$0A00-$0AFF	RS423 receive, and cassette workspace
$0B00-$0BFF	User defined function key definitions
$0C00-$0CFF	User defined character definitions
$0D00-$0D7F	Used by NMI routines (eg by Disc or Econet filing systems)
$0D80-$0DFF	allocated to machine operating system
$0E00	Default setting of PAGE
$4000-$7FFF	Optional RAM on Model B
$8000-$BFFF	One or more languages ROMS (e.g. BASIC, PASCAL)
$C000-$FBFF	Operating System ROM
$FC00-$FCFF	External memory mapped input/output (FRED)
$FD00-$FDFF	External memory mapped input/output (JIM)
$FE00-$FEFF	Internal memory mapped input/output (SHEILA)
$FF00-$FFFF	Operating System ROM

Zero Page Details

Zero Page Address	Purpose
$00-$6F	Basic OS
$70-$8F	Reserved for User
$90-$9F	Econet system.
$A0-$A7	Current NMI owner
$A8-$AF	Allocated for use by operating system commands
$B0-$BF	Filing system scratch space
$C0-$CF	Currently active filing system.
$D0	VDU status as returned by OSBYTE $75.
$D1	Byte mask for the current graphics point.
$D2-$D3	Text colour bytes to be ORed and EORed into memory
$D4-$D5	Similar in function to $D2 and $D3, only they are the graphics colour bytes
$D6-$D7	Address of the top line of the current graphics character cell (eight bytes)
$D8-$D9	Contain the address of the top scan line of the current text character.
$DA-$DF	Used as temporary workspace.
$E0-$E1	Used as a pointer to the row multiplication table, high bytes first.
$E2	Cassette filing system status byte
$E3	Cassette filing system options byte, as set by the *OPT command.
$E4-$E6	used as general operating system workspace.
$E7	Auto repeat countdown timer.
$E8-$E9	Pointer to the input buffer into which data is entered by OSWORD $01.
$EA	RS423 timeout counter
$EB	Cassette critical flag
$EC	Internal key number of the most recently pressed key, or zero if none is currently pressed.
$ED	Internal key number of the first key pressed of those still pressed.
$EE	Internal key number of the character to be ignored when scanning with OSBYTE $79.
$EF	Accumulator value for the most recent OSBYTE/ OSWORD.
$F0	X register value for the most recent OSBYTE/ OSWORD.
$F1	Y register value for the most recent OSBYTE/ OSWORD.
$F2-$F3	Text pointer for processing operating system commands and filenames.
$F4	RAM copy of the number of the currently selected paged ROM.
$F5	Current logical speech PHROM or ROM filing system ROM number
$F6-$F7	Used as an address pointer into a paged ROM or a speech PHrase ROM.
$F8-$F9	Not used
$FA-$FB	Used as general operating system workspace.
$FC	Used as an interrupt accumulator save register.
$FD-$FE	Points to the byte after the last BRK instruction
$FF	Escape flag.

Ram Usage

Address	Purpose
$0200-$0235	Vectors
$0236-$028F	Main system variables, accessed by OSBYTE calls $0A6 through $0FF.
$0290	VDU vertical adjust, as set by *TV (OSBYTE $090).
$0291	Interlace toggle flag, as set by *TV (OSBYTE $090).
$0292-$029B	Two stored values of the system clock, as read by �TIME�.
$029C-$02A0	Countdown interval timer value.
$02A1-$02B0	Paged ROM type table
$02B1-$02B2	INKEY countdown timer.
$02C5-$02C7	Used as work locations by OSWORD 1.
$02B6-$02B9	Low bytes of the most recent analogue converter values
$02BA-$02BD	High bytes of the most recent analogue converter values.
$02BE	Analogue system flag.
$02BF-$02C8	Event enable flags.
$02C9	Soft key expansion pointer.
$02CA	First auto repeat count.
$02CB-$02CD	Workspace for two key rollover processing.
$02CE	Sound semaphore.
$02CF-$02D7	Buffer busy flags.
$02D8-$02E0	Buffer start indices
$02E1-$02E9	Buffer end indices.
$02EA-$02EB	Block size of currently resident block of the open cassette input file.
$02EC	Block flag of the currently resident block of the open cassette input file
$02ED	Last character in currently resident block of the open cassette input file.
$02EE-$02FF	Used as an area to build OSFILE control blocks for LOAD and SAVE
$0300-$0301	Window Internal Co-ordinates: Left hand column in pixels.
$0302-$0303	Window Internal Co-ordinates: Bottom row in pixels.
$0304-$0305	Window Internal Co-ordinates: Right hand column in pixels.
$0306-$0307	Window Internal Co-ordinates: Top row in pixels.
$0308	Window in Absolute chars: Left hand column
$0309	Window in Absolute chars: Bottom row.
$030A	Window in Absolute chars: Right hand column.
$030B	Window in Absolute chars: Top row.
$030C-$030F	Current graphics origin in external co-ordinates.
$0310-$0313	Current graphics cursor in external co- ordinates.
$0314-$0317	Old graphics cursor in internal co- ordinates.
$0318	Text cursor X co-ordinate.
$0319	Text cursor Y co-ordinate.
$03lA	Line within current graphics character of the current graphics point.
$031B-$031E	Used either as graphics workspace or as the first part of the VDU queue.
$031F-$0323	VDU queue.
$0324-$0327	Current graphics cursor in internal co- ordinates.
$0328-$0349	General graphics co-ordinate workspace.
$034A-$034B	Text cursor position as an address as sent to 6845.
$034C-$034D	Text window width in bytes.
$034E	High byte of the address of the bottom of screen memory.
$034F	Number of bytes of memory taken up by a single character.
$0350-$0351	Address of the top left hand corner of the displayed screen, as is sent to the 6845.
$0352-$0353	Number of bytes taken per character row of the screen.
$0354	High byte of the size of the screen memory in bytes.
$0355	Current screen mode.
$0356	Memory map type.
$0357	Foreground text colour.
$0358	Background text colour.
$0359	Foreground graphics colour.
$035A	Background graphics colour.
$035B-$035C	Graphics plot mode for the foreground and background plotting respectively.
$035D-$035E	Used as a general jump vector.
$035F	Last setting of the 6845 cursor start register
$0360	Number of logical colours in the current mode minus one.
$0361	Number of pixels per byte minus one for the current mode, or zero if text only mode.
$0362-$0363	Left and right colour masks
$0364-$0365	X and Y co-ordinates of the text input cursor.
$0366	Character to be used as the output cursor in teletext mode
$0367	Font flag
$0368-$036E	Font location bytes.
$036F-$037E	Colour palette.
$0380-$039C	Used to store the header block for the BPUT file.
$039D	Offset of the next byte to be output into the BPUT buffer.
$039E	Offset of the next byte to be read from the BGET buffer.
$039F-$03A6	unused
$03A7-$03B1	Filename of the file being BGETed.
$03B2-$03BD	Block Read: Filename terminated by zero.
$03BE-$03C1	Block Read: Load address of the file.
$03C2-$03C5	Block Read: Execution address of the file.
$03C6-$03C7	Block Read: Block number of the block.
$03C8-$03C9	Block Read: Length of the block.
$03CA	Block Read: Block flag byte.
$03CB-$03CE	Block Read: Four spare bytes.
$03CF-$03D0	Block Read: Checksum bytes.
$03D1	Sequential block gap as set by *OPT 3.
$03D2-$03DC	Filename of the file being searched for. Terminated by zero.
$03DD-$03DE	Number of the next block expected for BGET.
$03DF	Copy of the block flags of the last block read.
$03E0-$03FF	Keyboard input buffer.
$0400-$07FF	Main workspace for the currently active language
$0800-$083F	Sound workspace.
$0840-$084F	Sound channel 0 buffer.
$0850-$085F	Sound channel 1 buffer.
$0860-$086F	Sound channel 2 buffer.
$0870-$087F	Sound channel 3 buffer.
$0880-$08BF	Printer buffer.
$08C0-$08FF	Envelope storage area, envelopes 1-4.
$0900-$09FF	Extended envelope/ RS523 output buffer /Casette Buffer
$0A00-$0AFF	Cassette input buffer/RS423 input buffer
$0B00-$0BFF	This page is the soft key buffer.
$0C00-$0CFF	Font for characters 224-255
$0D00-$0D9E	NMI routine.
$0D9F-$0DEF	Expanded vector set.
$0DF0-$0DFF	Paged ROM workspace storage locations. There is one byte per ROM.
$0E00-$7FFF	User Workspace (OSHWM)
$8000-$BFFF	Paged ROMs, including the BASIC and Disc Filing System ROMs.
$C000-$FFFF	Operating system ROM.
$FC00-$FEFF	Memory Mapped Peripherals
$FF00-$FFFF	Operating system ROM.

Video Registers
Write the register (R0-R18) 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
R0	Horizontal total	$7F
R1	Horizontal displayed characters	$50
R2	Horizontal sync position	$62
R3	Horizontal sync width/Vertical sync time	$28
R4	Vertical total	$26
R5	Vertical total adjust	$00
R6	Vertical displayed characters	$19
R7	Vertical sync position	$22
R8	Interlace/Display delay/Cursor delay	$01
R9	Scan lines per character	$30
R10	Cursor start line and blink type	$00
R11	Cursor end line	$08
R12	Screen start address H (Address /8)	$30
R13	Screen start address L (Address /8)
R14	Cursor position H
R15	Cursor position L
R16	Light pen position
R17	Light pen position
R18	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

Address	Section	Purpose	Details
$FE00	6845 CRTC Video controller	CRTC Reg Select	Select Register
$FE01	6845 CRTC Video controller	CRTC Reg Data	Change Register
$FE08	6850 ACIA Serial controller	Control Register / Status Register
$FE09	6850 ACIA Serial controller	Transmit data / Receive Data
$FE10	Serial ULA Serial system chip	Control Register
$FE20	Video ULA Video system chip	Video ULA Control	Select Screen Settings
$FE20	Video ULA Video system chip	ULA Palette	%LLLLAAAA L=Logical A=Actual color
$FE30	74LS161 Paged ROM selector	4 bit Rom select	%----RRRR
$FE40	6522 System VIA	Output B / Input B	Sound & Keyboard
$FE41	6522 System VIA	Output A / Input A
$FE42	6522 System VIA	Data Direction B
$FE43	6522 System VIA	Data Direction A
$FE44	6522 System VIA	T1 Low order Latches/Counter
$FE45	6522 System VIA	T1 High order Counter
$FE46	6522 System VIA	T1 Low order Latches
$FE47	6522 System VIA	T1 High order Latches
$FE48	6522 System VIA	T2 Low order Latches/Counter
$FE49	6522 System VIA	T2 High order Counter
$FE4A	6522 System VIA	Shift Register
$FE4B	6522 System VIA	Auxiliary Control Register
$FE4C	6522 System VIA	Peripheral Control Register
$FE4D	6522 System VIA	Interrupt Flags %v12ALSBK	V=systemVia 1=Timer1 (100hz) 2=Timer2 (speech) A=Analog conv L=Lightpen S=Shift reg B=Vblank K=Keypress
$FE4E	6522 System VIA	Interrupt Enable
$FE4F	6522 System VIA	Output A / Input A - No Handshake
$FE60	6522 User VIA	Output B / Input B
$FE61	6522 User VIA	Output A / Input A
$FE62	6522 User VIA	Data Direction B
$FE63	6522 User VIA	Data Direction A
$FE64	6522 User VIA	T1 Low order Latches/Counter
$FE65	6522 User VIA	T1 High order Counter
$FE66	6522 User VIA	T1 Low order Latches
$FE67	6522 User VIA	T1 High order Latches
$FE68	6522 User VIA	T2 Low order Latches/Counter
$FE69	6522 User VIA	T2 High order Counter
$FE6A	6522 User VIA	Shift Register
$FE6B	6522 User VIA	Auxiliary Control Register
$FE6C	6522 User VIA	Peripheral Control Register
$FE6D	6522 User VIA	Interrupt Flags
$FE6E	6522 User VIA	Interrupt Enable
$FE6F	6522 User VIA	Output A / Input A - No Handshake
$FE80	8271 FDC Floppy disc	Status Register / Command Register
$FE81	8271 FDC Floppy disc	Result Register / Parameter Register
$FE82	8271 FDC Floppy disc	Reset Register
$FE83	8271 FDC Floppy disc	unused
$FE84	8271 FDC Floppy disc	Read Data / Write Data
$FEA0	68B54 ADLC ECONET	Control Reg 1 / Status Reg 1
$FEA1	68B54 ADLC ECONET	Control Reg 2,3 / Status Reg 2
$FEA2	68B54 ADLC ECONET	Transmit FIFO (frame continue) / Receive FIFO
$FEA3	68B54 ADLC ECONET	Transmit FIFO (frame terminate) / Receive FIFO
$FEC0	uPD7002 ADC	Data latch and conversion start / Status Register
$FEC1	uPD7002 ADC	High Data Byte
$FEC2	uPD7002 ADC	Low Data Byte
$FEE0	Tube ULA	$FEE0- $FEFF

BBC Screen bit layout

In the chart below: Numbers are the bit number, A-D are the pixels contained in the byte.

The screen effectively uses 'nibble bitplanes' with bit 0 in the low nibble, and bit 1 in the high nibble (The reverse order to Amstrad CPC Mode 1)

4 color mode Bits/Pixels

In VRAM, 8 consecutive bytes go down the screen in an 8x4 block, the 9th byte 'jumps' back up to the top (effectively moving across the screen 4 pixels). Therefore there are two 8 byte vertical 'strips' in an 8x8 character block.

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
		USERV	$0200	Reserved
		BRKV	$0202	Break vector
		IRQ1V	$0204	All IRQ vector
		IRQ2V	$0206	Unrecognized IRQ vector
		CLIV	$0208	Interpret the command line given
OSCLI	$FFF7	�	�
OSBYTE	$FFF4	BYTEV	$020A	Perform miscellaneous OS operation using registers to pass parameters
OSWORD	$FFF1	WORDV	$020C	Perform miscellaneous OS operation control block to pass parameters
OSWRCH	$FFEE	WRCHV	$020E	Write character (to screen) from A
OSNEWL	$FFE7	�	�	Write LF,CR (&0A,&0D) to screen
OSASCI	$FFE3	�	�	Write a character (to screen) from A plus LF if (A)=&0D
OSRDCH	$FFE0	RDCHV	$0210	Read character (from keyboard) to A (Interrupts must be on! - CLI)
OSFILE	$FFDD	FILEV	$0212	Load or save a complete file
OSARGS	$FFDA	ARGSV	$0214	Load or save data about a file
OSBGET	$FFD7	BGETV	$0216	Load a single byte to A from file
OSBPUT	$FFD4	BPUTV	$0218	Save a single byte to file from A
OSGBPB	$FFD1	GBPBV	$021A	Load or save a block of memory to file
OSFIND	$FFCE	FINDV	$021C	Open or close a file
OSRDSC	$FFB9	�	�	Read byte from screen
OSWRSC	$FFB3	�	�	Write byte to screen
		FSCV	$021E	File system control entry
		EVNTV	$0220	Event interrupt
		UPTV	$0222	User print routine
		NETV	$0224	Used by Econet to take control of the computer.
		VDUV	$0226	Used by the VDU driver to direct unrecognised VDU 23 and PLOT commands.
		KEYV	$0228	Used by the operating system for all keyboard access. Enables the use of external keyboards.
		INSV	$022A	Insert into buffer vector.
		REMV	$022C	Remove from buffer vector.
		CNPV	$022E	Count / purge buffer vector
		IND1V	$0230	Spare vector.
		IND2V	$0232	Spare vector.
		IND3V	$0234	Spare vector.

VDU Codes
These are special character sequences that can be sent to the OS Print Character routine (OSASCI - $FFE3)
Some take multiple additional byte parameters (+Bytes).

Dec	Hex	CTRL	+bytes	Function
0	0	�	0	Does nothing
1	1	A	1	Send next character to printer only
2	2	B	0	Enable printer
3	3	C	0	Disable printer
4	4	D	0	Write text at text cursor
5	5	E	0	Write text at graphics cursor
6	6	F	0	Enable VDU drivers
7	7	C	0	Make a short bleep (BEL)
8	8	H	0	Move cursor back one character
9	9	I	0	Move cursor forward one character
10	A	J	0	Move cursor down one line
11	B	K	0	Move cursor up one line
12	C	L	0	Clear text area
13	D	M	0	Carriage return
14	E	N	0	Paged mode on
15	F	O	0	Paged mode off
16	10	P	0	Clear graphics area
17	11	Q	1	Define text colour
18	12	R	2	Define graphics colour
19	13	S	5	Define logical colour
20	14	T	0	Restore default logical colours
21	15	U	0	Disable VDU drivers or delete current line
22	16	V	1	Select screen MODE
23	17	W	9	Re-program display character
24	18	X	8	Define graphics window
25	19	Y	5	PLOT K,X,Y
26	1A	Z	0	Restore default windows
27	1B	[	0	ESCAPE value
28	1C	\	4	Define text window
29	1D	]	4	Define graphics origin
30	1E	*	0	Home text cursor to top left of window
31	1F	_	2	Move text cursor to X,Y
127	7F	DEL	0	Backspace and delete

OSBYTEs

A=Routine Number... X,Y=Parameters
The OSBYTE Call address is $FFF4 - it is Indirected through $020A

Dec	Hex	Purpose
0	0	Print operating system version
1	1	User OSBYTE call, read
2	2	Select input stream
3	3	Select output stream
4	4	Enable
5	5	Select printer destination
6	6	Set character ignored by printer
7	7	Set RS423 baud rate for receiving data
8	8	Set RS423 baud rate for data transmission
9	9	Set flashing colour mark state duration
10	A	Set flashing colour space state duration
11	B	Set keyboard auto-repeat delay interval
12	C	Set keyboard auto-repeat rate
13	D	Disable events
14	E	Enable events
15	F	Flush selected buffer class
16	10	Select ADC channels to be sampled
17	11	Force an ADC conversion
18	12	Reset soft keys
19	13	Wait for vertical sync
20	14	Explode soft character RAM allocation
21	15	Flush specific buffer
117	75	Read VDU status
118	76	Reflect keyboard status in LEDs
119	77	Close any SPOOL or EXEC files
120	78	Write current keys pressed information
121	79	Perform keyboard scan
122	7A	Perform keyboard scan from 16 (&10)
123	7B	Inform OS, printer driver going dormant
124	7C	Clear ESCAPE condition
125	7D	Set ESCAPE condition
126	7E	Acknowledge detection of ESCAPE condition
127	7F	Check for EOF on an open file
128	80	Read ADC channel or get buffer status
129	81	Read key with time limit
130	82	Read machine high order address
131	83	Read top of OS RAM address (OSHWM)
132	84	Read bottom of display RAM address (HIMEM)
133	85	Read bottom of display address for a given MODE
134	86	Read text cursor position (POS and VPOS)
135	87	Read character at cursor position
136	88	Perform *CODE
137	89	Perform *MOTOR
138	8A	Insert value into buffer
139	8B	Perform *OPT
140	8C	Perform *TAPE
141	8D	Perform *ROM
142	8E	Enter language ROM
143	8F	Issue paged ROM service request
144	90	Perform *TV
145	91	Get character from buffer
146	92	Read from FRED, 1 MHz bus
147	93	Write to FRED, 1 MHz bus
148	94	Read from JIM, 1 MHz bus
149	95	Write to JIM, 1 MHz bus
150	96	Read from SHEILA, mapped I/ O
151	97	Write to SHEILA, mapped I/ O
152	98	Examine buffer status
153	99	Insert character into input buffer
154	9A	Write to video ULA control register and copy
155	9B	Write to video ULA palette register and copy
156	9C	Read/ write 6850 control register and copy
157	9D	Fast Tube BPUT
158	9E	Read from speech processor
159	9F	Write to speech processor
160	A0	Read VDU variable value
166	A6	Read start address of OS variables (low byte)
167	A7	Read start address of OS variables (high byte)
168	A8	Read address of ROM pointer table (low byte)
169	A9	Read address of ROM pointer table (high byte)
170	AA	Read address of ROM information table (low byte)
171	AB	Read address of ROM information table (high byte)
172	AC	Read address of key translation table (low byte)
173	AD	Read address of key translation table (high byte)
174	AE	Read start address of OS VDU variables (low byte)
175	AF	Read start address of OS VDU variables (high byte)
176	B0	Read/ write CFS timeout counter
177	B1	Read/ write input source
178	B2	Read/ write keyboard semaphore
179	B3	Read/ write primary OSHWM
180	B4	Read/ write current OSHWM
181	B5	Read/ write RS423 mode
182	B6	Read character definition explosion state
183	B7	Read/ write cassette/ ROM filing system switch
184	B8	Read RAM copy of video ULA control register
185	B9	Read RAM copy of video ULA palette register
186	BA	Read/ write ROM number active at last BRK (error)
187	BB	Read/ write number of ROM socket containing BASIC
188	BC	Read current ADC channel
189	BD	Read/ write maximum ADC channel number
190	BE	Read ADC conversion type
191	BF	Read/ write RS423 use flag
192	C0	Read RS423 control flag
193	C1	Read/ write flash counter
194	C2	Read/ write mark period count
195	C3	Read/ write space period count
196	C4	Read/ write keyboard auto-repeat delay
197	C5	Read/ write keyboard auto-repeat period
198	C6	Read/ write *EXEC file handle
199	C7	Read/ write *SPOOL file handle
200	C8	Read/ write ESCAPE, BREAK effect
201	C9	Read/ write Econet keyboard disable
202	CA	Read/ write keyboard status byte
203	CB	Read/ write RS423 handshake extent
204	CC	Read/ write RS423 input suppression flag
205	CD	Read/ write cassette/ RS423 selection flag
206	CE	Read/ write Econet OS call interception status
207	CF	Read/ write Econet OSRDCH interception status
208	D0	Read/ write Econet OSWRCH interception status
209	Dl	Read/ write speech suppression status
210	D2	Read/ write sound suppression status
211	D3	Read/ write BELL channel
212	D4	Read/ write BELL envelope number/ amplitude
213	D5	Read/ write BELL frequency
214	D6	Read/ write BELL duration
215	D7	Read/ write startup message and !BOOT options
216	D8	Read/ write length of soft key string
217	D9	Read/ write number of lines printed since last page
218	DA	Read/ write number of items in VDU queue
219	DB	Read/ write TAB character value
220	DC	Read/ write ESCAPE character value
221	DD	Read/ write character &CO to &CF status
222	DE	Read/ write character &DO to &DF status
223	DF	Read/ write character &EO to &EF status
224	E0	Read/ write character &FO to &FF status
225	El	Read/ write function key status
226	E2	Read/ write SHIFT+ function key status
227	E3	Read/ write CTRL+function key status
228	E4	Read/ write CTRL+SHIFT+function key status
229	E5	Read/ write ESCAPE key status
230	E6	Read/ write flags determining ESCAPE effects
231	E7	Read/ write JRQ bit mask for user 6522
232	E8	Read/ write IRQ bit mask for 6850
233	E9	Read/ write IRQ bit mask for system 6S22
234	EA	Read flag indicating Tube presence
235	EB	Read flag indicating speech processor presence
236	EC	Read/ write write character destination status
237	ED	Read/ write cursor editing status
238	EE	Read/ write location &27E, not used by 05 1.20
239	EF	Read/ write location &27F, not used by 05 1.20
240	F0	Read/ write location &280, not used by 05 1.20
241	F1	Read/ write location &281, used by *FX 1
242	F2	Read RAM copy of serial processor ULA
243	F3	Read/ write timer switch state
244	F4	Read/ write soft key consistency flag
245	F5	Read/ write printer destination flag
246	F6	Read/ write character ignored by printer
247	F7	Read/ write first byte of BREAK intercept code
248	F8	Read/ write second byte of BREAK intercept code
249	F9	Read/ write third byte of BREAK intercept code
250	FA	Read/ write location &28A, not used by OS1.20
251	FB	Read/ write location &28B, not used by OS1.20
252	FC	Read/ write current language ROM number
253	FD	Read/ write last BREAK type
254	FE	Read/ write available RAM
255	FF	Read/ write start up options

Interrupt Vectors

The standard 6502 interrupt vectors from $FFFA+ are ROM, however these jump to vectors in low memory addresses (Jumps to the address at...)

IRQ and BRK interrupts back up A at address $00FC.

NMI is different, it causes a jump to address $0D00 - the firmware puts an RTI command here during reset.

Vector Address	Remaps to	Function	Registers stored by Firmware
$FFFE-$FFFF	Vector at $0202-$0203	BRK	STA $FC
$FFFE-$FFFF	Vector at $0204-$0205	IRQ1 (Primary IRQ)	STA $FC
$FFFE-$FFFF	Vector at $0206-$0207	IRQ2 (Unrecognised IRQ)	STA $FC
$FFFA-$FFFB	Jumps to $0D00	NMI

We need to clear the interrupt, we can do this by writing #255 to $FE4D

;$FE4D Interrupt flag register- %7654321
; 7 = IRQ
; 6 = Timer 1
; 5 = Timer 2
; 4 = CB1 Active Edge
; 3 = CB2 Active Edge
; 2 = Shift Reg complete 8 shifts
; 1 = CA1 Active Edge
; 0 - CA2 Active Edge

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

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