YAGNI

The following article was written by Roger Cullis, and published in PRACTICAL COMPUTING February 1985, Pages 122- 125. The text below was OCR’ed from the magazine.

The original article was accompanied by the following notice:

Copyright

This article summarises the operation of the BBC Micro and where the routines are to be found, and is intended to help users to write programs which interact fully with the built-in software. But you shduld remember that the source code and object code of ROM routines are the subject of copyright and may not be used without the copyright owner’s permission. Although you may freely call them from programs running on the computer, you cannot extract or copy them for your own software.

The main text of the article was as follows

Inside Os: Roger Cullis dissects the BBC Micro’s operating-system ROM routines.

IN ORDER to write software which makes full use of the facilities provided by a computer, it is vitally important to understand the computer’s components and how they are controlled. The BBC Micro is a complex machine which not only drives a wide variety of peripheral devices through built-in interfaces but can also be connected to an increasing number of second processors to enhance its computing power. In many cases these resources can only be exploited by direct access to the utilities and routines which are an integral part of the languages and operating system provided in the machine’s firmware.

The BBC Micro is built round the Mostek 6502 eight-bit microprocessor. In its standard form it has 32K of ROM and either 16K of RAM as the model A, or 32K as the model B. It was designed as an expandable system and so is primarily an I/O device. It can service a variety of peripheral devices such as serial and parallel printers, local area networks, disc drives and speech synthesisers. For this reason blocks of memory hake to be reserved for specific applications. For example, in order to provide buffers for transfer of data between the computer and the floppy-disc controller, nearly 3K of read/write memory is required. Adding Econet takes another 0.5K, and speech
synthesis a further 1K.

Memory map

The 6502 microprocessor has a 16-bit address bus, which means that it can access a 216 or 64K memory bank. Of this, 32K is ROM and memory-mapped I/O and, on the model B, 32K is RAM — 16K on the model A. The arrangement of memory is shown in figure 1. RAM occupies the lower 32K addresses from &0000-7FFF and ROM occupies two 16K blocks from &8000-BFFF and &C000-FFFF. The lower ROM block is allocated to utilities, including Basic, the disc filing system, word processors such as View, or graphics extensions.

Up to 16 different 16K ROMs can be connected to this address space, although only one may be active at a given time. The machine operating system or a control program ensures that the appropriate page ROM is switched in when it is required. This operating system, which controls the running of the computer, occupies the upper ROM block. &C000-FFFF.

Most of the memory space contains operating code, but part of the address allocation is devoted to memory-mapped I/O. If an instruction is written to one of these addresses, it is not stored but passed directly to the peripheral device to which the address is allocated.

The OS, the paged ROMs and I/O devices all require read/write memory to store data during operation. This memory may be set aside exclusively for the purpose, as is the case with the soft-key buffer, &0B00-0BFF, which holds the strings called up by the user-definable keys. Alternatively, it may be shared among several chips, as with the paged ROMs’ public workspace or the filing system zero-page locations, or it may be allocated dynamically during the operation of a program. An example of this dynamic allocation is found in the Basic stack, which acts as a temporary store, and expands and contracts according to the needs of the program.

The screen buffer where the computer stores what is currently displayed on the screen occupies the highest region of RAM. Its size depends on the mode which is being displayed. Mode 7, Teletext mode, occupies only 1K, whole mode 0 requires 20K.

Page 0, addresses &0000-00FF, is used for dynamic storage during operation, and page 1, &0100-01FF, is allocated to the 6502 stack. The operating system takes pages 2 and 3; pages 4 to 7 are given to the current language ROM, or to interfacing routines if a second processor is active. Pages 8 to C, addresses &0800-0CFF, are devoted to operating system buffers for input and output and for storage of soft- key definitions and sound envelopes. Page D, addresses &0D00-0DFF, contains the non-maskable interrupt routines and parameters such as the current paged ROM status and extended vectors.

The operating system ROM contains the routines to start up the computer when it is switched on or reset by pressing the Break key. It also supervises the chips which control the peripheral devices and handles the flow of data to and from them.

The BBC Micro can have a variety of different filing systems, such as disc, Econet and Telesoft, for data storage and retrieval. Mostly these are in the form of separate paged ROMs, but the two simplest are built into the OS ROM. They are the ROM filing system, which can be used for data retrieval only, and the cassette filing system, which only implements a limited subset of the full filing- system operations.

OS routines

There have been several versions of the OS, commencing with version 0.1. Each successive version has implemented more features, culminating in the current OS 1.2 which has remained unchanged for some time. The general layout of the operating system is shown in the memory map. There are several well-defined features which serve quite independent functions.

The computer is able to control a wide variety of peripheral devices by using dedicated peripheral controller chips. For instance, disc storage is controlled by an Intel 8271 floppy-disc controller chip, Econet by a Motorola 6854 advanced data link controller, and the VDU by a Motorola 6845 cathode-ray tube controller. These chips each have several registers which need to be initialised, and they expect instructions in a pre-defined format.

The look-up tables and control routines to meet these needs are stored in the operating-system ROM or, in the case of filing systems, in a separate paged ROM. The first part of the OS is devoted to look- up tables of parameters required to set up and run the visual display, starting with the character fount look-up table at &C000-C0BF.

Text characters are displayed on an eight by eight matrix. The fount for each character is stored in ascending ASCII order as an eight-byte block, each byte representing one line of the matrix. In each byte, a bit set to I represents a dot that is to be displayed. For example, the symbol +, ASCiI code 2B, held in ROM at &C058-C05F, is displayed as shown in table 1.

&C300 contains an unconditional jump to VDU initialisation routines, and at &C303 there are the BRK embedded messages. &C31F-C332 contains the byte mask look-up tables used in processing pixels for graphics display.

&C333-C353 and &C354-C374 contain the two-byte address of the entry point of the VDU routines, indexed by VDU code. The high byte held in &C354-C374, is modified so that the number of associated parameters can be derived from it. The parameters are then assembled in a VDU queue. The low byte is held in &C333-&C353.

&C375-C3E6 holds the *640 and *40 multiplication tables, which determine the number of characters from the origin in text display modes. &C3E7-C3F6 holds look-up tables for default text window size, indexed by screen mode.

&C3F7-C447 contains look-up tables for various display parameters. Several of these tables overlap one another, and some serve a dual purpose. &C441-C444 holds the sound pitch offset. If more than one channel is programmed to output the same frequency, it is not possible to distinguish by ear that a second or third channel is active. To avoid this effect, the pitch of channels 2 and 3 is offset slightly in the same way that piano strings are slightly detuned. &C447-C458 holds the screen buffer size and location look-up tables. &C463-C468 holds the character position determination look-up tables. &C469-C4A9 holds the initial register settings for 6845 CRTC chip.

&C4AA-C4B5 holds the look-up tables for alternative entry points and branch loops in VDU processing routines. &C4B6-C4B9 holds the character definitions for the non-standard ASCII codes used in mode 7. The soft-character RAM allocation is done by &C4BA-C4BF.

The VDU routines are held in &C4CO-&D93F, starting with the VDU command handler. Current VDU status is stored as a series of flag bits at zero-page location &D0. On input of a VDU command, permissible action is checked against VDU status and the entry point of the associated routine ascertained from a look-up table.

The value of the status bit may be determined with Osbyte 75. The significance of the bits is
0 — enable printer
1 — disable scrolling
2 — enable paged mode
3 — enable software scrolling
5 — enable text at graphics cursor
6 — enable cursor editing
7 — disable VDU

The command is processed and converted into instructions which are stored in corresponding registers of the 6845 CRTC to modify the display. Parameters associated with a VDU command, such as VDU25,k,X,Y are stored as a VDU queue and processed in sequence.

Graphics display is controlled by manipulation of graphics co-ordinates. A region of low RAM is set aside as graphics co-ordinate workspace. Values such as the previous position of the graphics cursor are stored here. It is also used, for example, as a co-ordinate store in the Plot and Fill routines.

Colour processing is controlled by a palette which stores the relationships between logical and physical colours. It is located in the video ULA chip.

The page 2 setup default values, held at &D940-D9CC, are copied by the Reset routine to page 2.

This article forms part of a book to be published by Losco Ltd, P0 Box 4, Cranleigh, Surrey GU6 8BQ, price £11.95 plus £1.50 post and packing. A further instalment will appear in next month’s Practical Computing.

Unfortunately, I do not have a copy of the follow-up article. I believe this book to have been “Within the BBC Microcomputer” published in 1985. ISBN:0948203005.

The article was accompanied by the following table. Unfortunately, the font used in the table of OS entry points failed to OCR accuratlely. I have made a cursory attempt to clean up the text by hand, but I don’t doubt that many errors remain:

VDU display setup parameters and routines.
C000	C2FF	character font lookup table
C300		initialise system
C303		‘BBC Computer’ embedded message
C312		‘16K + BELL’ embedded message
C317		‘32K + DELL’ embedded message
C31F	C32E	4-colour MODE byte mask lookup table
C32F	C332	16-colour MODE byte mask lookup table
C333	C353	VDU entry point lo lookup table
C354	C374	VDU entry point hi parameter lookup table
C375	C383	*640 multiplication table (40-,80-column MODEs)
C385	C3E5	*40 multiplication table (teletext MODE)
C3E7	C3EE	text window – bottom row lookup table
C3EF	C3F6	text window – right-hand column lookup table
C3F7	C3FE	video ULA control register setting
C3FF	C406	number of bytes per character for each display MODE
C407	C408	mask table for 2-colour MODEs
C409	C40C	mask table for 4-colour MODEs
C40D	C414	mask table for 2-colour MODEs, font flag mask table
C4l4	C41B	number of colours minus one for each MODE
C418	C425	6COL PLOT options processing lookup table
C424	C425	2-colour MODE colour parameter lookup table
C426	C429	4-colour MODE colour parameter lookup table
C42A	C439	16-colour MODE colour parameter lookup table
C430	C44l	display MODE pixels/byte -l lookup table
C440	C447	screen display memory index lookup table
C44l	C444	sound pitch offset by channel lookup table
C44B	C44E	CRTC setup parameter
C44F	C453	CRTC setup parameter
C447	C458	VDU section control numbers
C459	C4SD	MSB of memory occupied by screen buffer
C45E	C462	MSB of first location occupied by screen buffer
C463	C465	number of bytes/row
C466	C468	row multiplications table pointer lo lookup table
C469	C46D	CRTC cursor end register setting lookup table
C46E	C479	6845 registers 0-11 for MODEs 0-2
C47A	C485	6845 registers 0-11 for MODE 3
C486	C49l	6845 registers 0-11 for MODEs 4-5
C492	C49D	6845 registers 0-11 for MODE 6
C4A93	C4A9	6845 registers 0-11 for MODE 7
C4AA	C4AD	VDU routine vector addresses
C4AE	C4B1	VDU routine branch vector address lo
C4B2	C4B5	VDU routine branch vector address hi
C4B6	C4B9	teletext character conversion table
C4BA	C4BF	soft character RAM allocation
VDU routines.
C4C0		check VDU command, if normal character, PRINT it
C4ED		RUBOUT
C511		VDU 00 – do nothing
C511		VDU 06 – enable VDU drivers
C511		YDU 27 – do nothing
C53B		VDU 01 – send next character to printer only
C55E		confirm input, output cursors not separated
C565		prepare cursor character for display
C568		interchange cursors, set CRTC address registers, toggle VDU screen status
C588		get graphics cursor printing status bit
C58D		YDU 14 – paged mode on
C596		YDU 02 – enable printer
C59B		VDU 21 – disable VDU drivers or delete current line
C59D		set VDU status or, if zero, reset default
C5A1		VDU 03 – disable printer
C5A6		YDU 15 – paged mode off
C5A8		reset specified VDU status bits
C5AD		VDU 04 – write text at text cursor
C5B9		VDU 05 – write text at graphics cursor
C5C5		VDU 08 – backspace one character
C621		move graphics cursor one position
C65B		VDU 11 – move cursor up one line
C664		VDU 09 – advance tent cursor if enabled
C684		set text cursor to new line, scrolling if required
C6AC		clear text line to background, calculate offset, set CRTC address
C6AF		calculate offsets of character row, set text cursor address register
C6B4		paged mode scrolling
C6F0		VDU 10 – move cursor down one line (line feed)
C6FA		VDU 28 – define test window
C735		OSWORD 09 – read pixel value
C74B		OSWORD 0B – read palette
C759		VDU 12 – clear test area
C779		VDU 30 – home text cursor to top left
C787		VDU 31 – move test cursor to 0,9
C7AB		interchange current tent cursor with value from general graphics coordinate workspace
C7AF		VDU 13 – move cursor to start of current line
C7C0		VDU 16 – clear graphics area
C7F9		VDU 17 – define text colour
C7FD		VDU 18 – define graphics colour
C839		VDU 20 – restore default logical colour
C88F		save colour index and define logical colour
C892		VDU 19 – define logical colour
C89E		set logical/physical colour relationship
C8E0		OSWORD OC – write palette
C8EB		VDU 22 – select screen MODE
C8F1		VDU 23 – re-program display character or write to CRTC
C938		process unrecognised VDU command
C93F		setup CRTC
C951		set 6845 cursor register to previous value
C954		set 6845 cursor parameters register = A-register
C958		write parameter from VDU queue to specified CRTC register
C95E		set specified CRTC register
C98C		VDU 25 – PLOT k,X,Y
C994		backspace one row, set CRTC screen start address registers
C9A4		advance one row, set CRTC screen start address registers
C9B3		set 6845 CRTC screen start address registers
C9BD		VDU 26 – restore default windows
C9F6		save 6845 test cursor position address, set corresponding 6845 registers
CA02		set 6845 text cursor address registers
CA0E		set 6845 CRTC registers indexed by 9-register
CA2B		write two bytes to consecutive 6845 registers
CA39		VDU 24 – define graphics window
CA88		set test window width according to character size
CAA2		VDU 29 – define graphics origin
CAAC		VDU 127 – backspace and delete
CAD4		add number of bytes in row to display address
CAE0		zero POINT line counter and scroll if enabled
CAE3		scroll if enabled
CB14		zero page PRINT line counter
CB1D		initialise video display parameters and workspace
CB33		set up video display parameters table
CBC1		initialise screen display parameters
CBF3		OSWORD 0A – read character definition
CC02		character font definition indirection vector lookup table
CCF5		process unrecognised VDU command
CCF8		scroll back one row, check if wrap-round seeded
CD07		OSBYTE 14 – explode soft character ROM allocation
CD3F		move text cursor to next line
CD6A		process character for display
CD77		recover A-register, processor status from stack
CD7A		process cursor character for display
CDA4		soft scroll one line
CDCE		set scan line address, scroll back until counter zero
CDDA		interchange current test cursor with value from coordinate workspace
CDDE		interchange two Page 3 bytes indexed by X,Y
CDE2		interchange old and current graphics cursors
CDE4		interchange old graphics cursor with value from coordinate workspace
CDE6		interchange four-byte block indexed by X,Y in Page 3
CDFF		scroll text one line
CE2A		set scan line address source line address
CE38		copy source line to scan line
CE5B		determine test window height, if non-zero do CR
CE6E		set text cursor U-coordinate to left-hand side of text window
CE73		add character to scan line
CEE8		confirm cursor in text window, calculate character row offset
CF06		calculate address of text cursor
CF63		PRINT character at graphics cursor
C046		home graphics cursor
CFAD		set graphics cursor X-coordinate to left-hand column
CFB7		set up character definition and display character
CFBF		display character
CFDC		insert teletext character in scan line, convert if required
D03E		set up character definition pointers
D060		process PLOT command
D0B3		set Page 0 colour processing bytes
D0D9		move graphics cursor to absolute position
D0DC		copy current graphics cursor coordinates from VDU queue
D0DE		copy four bytes from VDU queue
D0E3		set colour of point at current graphics cursor
D0EB		omit last paint in line
D0F0		set colour of current graphics point, save to graphics scan line
D0F3		process colour and save to graphics scan line
D104		process colour of current graphics point
D10D		check current graphics cursor position with respect to to window boundaries
D10F		check position of indexed graphics coordinates with respect to window boundaries
D128		check current graphics point position with respect to window boundaries
D149		adjust coordinates according to display MODE
D14D		adjust PLOT coordinate according to display options
D176		adjust coordinate for relative/absolute PLOT, divide it by 2
D1AD		divide PLOT coordinate by 2
D1B8		calculate graphics cursor offset according to display MODE
D1D5		adjust cursor coordinates, calculate cursor offset
D1E3		use graphics origin to calculate cursor offset
D1ED		execute PLOT command
D214		set up branch loops, PLOT parameters
D2E3		set scan byte if appropriate, adjust coordinates and scan position
D3D3		decrement graphics cell top line address by one raw, set Y=7
D3ED		set graphics cask left-hand colour cask, increment graphics cell by use line
D3FD		set graphics mask right-hand colour cask, decrement graphics cell by one line
DA0D		calculate graphics cursor relative to graphics window
D411		calculate graphics coordinates relative to specified paint
D418		calculate graphics coordinate offset
D42C		if necessary, set up working coordinates, evaluate difference,
D42C		make positive, divide Y by 2
D459		take modulus of working coordinate, divide Y coordinate by 2
D467		change sign of working coordinate, if negative
D47C		copy 8 Page 3 bytes to new location (indexed by X,Y)
D480		copy two Page 3 bytes to coordinate workspace
D482		copy two Page 3 bytes to new location (indexed by X,Y)
D486		copy current graphics cursor to coordinate workspace
D488		copy current graphics cursor to new Page 3 location (indexed by Y)
D48A		copy four Page 3 bytes to new location (indexed by X,Y)
D49B		fore 2s complement of number in Y-,A-registers
D4AA		if byte legal, process colour, else, discard it and cove cursor
D4BF		omit last point in inverting actions
D506		PLOT with dotted line
D545		move graphics cursor and calculate offset for MODE
D548		get current graphics byte cask, set colour, save byte to scan line
D574		reset graphics byte and save to scan line
D592		check X coordinate from VDU queue against graphics window boundary
D5AC		process working I coordinate, transfer to YOU queue
D5CC		OSWORD 0D – read last two graphics cursor positions
D5DE		OSWORD 0D – read old and current graphics position
D5D5		transfer four bytes index by 4-register to OS buffer
D5EA		PLOT and fill triangle
D632		11 old graphics cursor greater than new PLOT point, interchange thee
D636		of source coordinate greater than destination coordinate, interchange points
D647		OSBYTE 86 – read text cursor position (POS and VPOS)
D658		process coordinates and PLOT points til target reached
D6A2		set and arrange working coordinates…
D6A6		arrange working coordinates, process colour mask, save to scan line
D70E		recover target coordinates from stack
D774		derive coordinates for next point in triangular PLOT
D7AC		add stored value to working coordinate, increment counter if necessary
D7C2		OSBYTE 87 – read character at text cursor position
D808		derive character definition from displayed character
D839		determine logical colour of specified point
D85D		get coordinates from YOU queue, set up graphics line address, colour mask
D85F		set coordinate source index, set up line address, colour mask
D864		set up graphics line address
D8CE		if enabled, PRINT at cursor position
D905		if test cursor enabled, read character
D918		reset CRTC cursor register, cursors together, enable screen, set 4-register to 80
D923		OSBYTE 85 – read HIMEM
8926		OSBYTE 85 – read address of screen buffer for MODE
Page 2 setup – default values.
D940	D976	Page 2 vectors default settings
D976	D9CC	MOS variables default settings

I drew these diagrams years ago, whilst seeking to understand the various sub-systems of the BBC Micro.

The BBC Micro System VIAThe BBC Micro Keyboard