MOS Memory Usage on the Master

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Memory Locations used by MOS Vn. 3.20

The following is a list of the locations used by the Master MOS. The information is (I believe) correct for Version 3.20 of the MOS, ie the most common version. I suspect that it will not be very different for other versions of the Master MOS. Some locations certainly are different in the MOS versions used by the Model B and B+.

(All addresses in the following are in hex.)

0 - 8F		Zero-page workspace for the current language, or for the Tube host code if the Tube is active. (When BASIC is the current language it only uses locations 0 to 6F, leaving 70 to 8F available for user programs.)
90 - 9F		Econet workspace. (Available to user programs on non-Econet machines.)
A0 - A7		NMI workspace. Usable only by filing systems which can claim NMIs
A8 - AF		General "*" command workspace.
B0 - CF		Filing system workspace. The locations between C0 and CF must be preserved between filing system calls.
D0 - FF		MOS zero page space:
		D0		VDU status byte (see documentation for OSBYTE &75)
		D1		Byte mask for current graphics point
		D2		Text colour "or" mask
		D3		Text colour "eor" mask
		D4		Graphics colour "or" mask
		D5		Graphics colour "eor" mask
		D6,D7		Address of graphics character cell
		D8,D9		Address of text character cell
		DA-E1		Temporary workspace for VDU driver
		E2		CFS status byte
		E3		CFS *OPT byte
		E4-E6		Miscellaneous workspace
		E7		Keyboard auto-repeat countdown timer
		E8,E9		Address of input buffer being used by OSWORD 0
		EA		Serial port timeout counter
		EB		CFS "critical" flag
		EC		Internal key number of last key-press
		ED		Internal key number of last-but-one key-press
		EE		RAM copy of 1MHz bus paging register
		EF		OSBYTE / OSWORD A value
		F0		OSBYTE / OSWORD X value
		F1		OSBYTE / OSWORD Y value
		F2,F3		Address pointer (with Y register) of rest of entered command line
		F4		RAM copy of ROMSEL register
		F5-F7		ROM Filing System 3-byte pointer
		F8,F9		Unused
		FA,FB		General workspace
		FC		Stored value of A-register during interrupts
		FD,FE		Address of byte after last BRK instruction encountered
		FF		Escape flag
100 - 1FF		Error message buffer, Tube OS call control block and CPU stack. (The lower part from location 100 upwards is used for error messages by programs in Sideways memory. The middle part (from location 128) is used if the Tube is active as a host control block for OS calls. The upper part is the CPU stack, growing down from location 1FF.)
200 - 235		Main vectors:
		200,201		USERV – User vector, used with *CODE and *LINE
		202,203		BRKV – BRK vector
		204,205		IRQ1V – Main interrupt vector
		206,207		IRQ2V – Secondary interrupt vector
		208,209		CLIV – OSCLI indirection
		20A,20B		BYTEV – OSBYTE indirection
		20C,20D		WORDV – OSWORD indirection
		20E,20F		WRCHV – OSWRCH indirection
		210,211		RDCHV – OSRDCH indirection
		212,213		FILEV – OSFILE indirection
		214,215		ARGSV – OSARGS indirection
		216,217		BGETV – OSBGET indirection
		218,219		BPUTV – OSBPUT indirection
		21A,21B		GBPBV – OSGBPB indirection
		21C,21D		FINDV – OSFIND indirection
		21E,21F		FSCV – Filing System Control Vector
		220,221		EVENTV – Event vector
		222,223		UPTV – User printer vector
		224,225		NETV – Econet vector
		226,227		VDUV – VDU vector
		228,229		KEYV – Keyboard access vector
		22A,22B		INSV – Buffer insert vector
		22C,22D		REMV – Buffer remove vector
		22E,22F		CNPV – Buffer count / purge vector
		230,231		IND1V – Spare vector 1
		232,233		IND2V – Spare vector 2
		234,235		IND3V – Spare vector 3
236 - 28F		Main MOS Variables (as accessed by Osbytes A6 to FF):
		236,237		Start address of MOS variables
		238,239		Address of ROM pointer table
		23A,23B		Address of ROM information table
		23C,23D		Address of keyboard translation table
		23E,23F		Address of VDU variables origin
		240		CFS timeout counter
		241		Input source (0 - keyboard, 1 - RS423)
		242		Keyboard semaphore (0 - disabled, FF - enabled)
		243		ROM polling semaphore
		244		OSHWM (MSB only)
		245		RS423 interpretation status (0 - as keyboard, 1 - default)
		246		NOIGNORE status (bit 7 set if no ignore character)
		247		CFS/ROM filing system switch (0 - CFS, 1 - ROM)
		248		RAM copy of video ULA control register
		249		RAM copy of video ULA palette register
		24A		Sideways memory slot active at last BRK
		24B		Sideways memory slot containing BASIC
		24C		Current ADC channel number
		24D		Maximum ADC channel number
		24E		ADC conversion type (0 or C - 12 bits, 8 - 8 bits)
		24F		RS423 busy flag (bit 7 set if busy)
		250		RAM copy of serial ACIA control register
		251		Flash counter
		252		First colour duration
		253		Second colour duration
		254		Keyboard auto-repeat delay (0 for no repeat)
		255		Keyboard auto-repeat rate
		256		*EXEC file handle
		257		*SPOOL file handle
		258		Break and Escape effect (bit 0 set - Escape disabled, bit 1 set - memory will be cleared on Break)
		259		Keyboard status (0 - enabled, non-0 - disabled)
		25A		Keyboard shift/ctrl status byte (see documentation for OSBYTE &CA)
		25B		RS423 input buffer minimum space
		25C		RS423 ignore flag (0 - input enabled, non-0 - disabled)
		25D		RS423 destination (0 - to RS423, 1 - to cassette)
		25E		Econet OS call interception status
		25F		Econet input interpretation status
		260		Econet output interpretation status
		261		Reserved (This location was used for speech suppression status on Model B)
		262		Sound suppression status (0 - sound enabled, non-0 - disabled)
		263-266		Bell (VDU 7) information (channel, ampl/env., frequency, duration)
		267		Startup message and !BOOT error action status. Note: There is an error in the description of OSBYTE D7 in the Reference Manual.    bit 0    !BOOT error status. 0 - lock up on ROM error; 1 - lock up on disk error   bit 7   Start up message status. 0 - suppress message; 1 - normal message
		268		Length of soft key string
		269		Paged mode line count
		26A		Length of VDU queue
		26B		Tab key code
		26C		Escape character
		26D-270		Interpretation of input values C0 to FF (values not normally generated by keypresses). Each byte controls a block of &10 values. Meanings: 0 - ignore; 1 - use soft key expansion; 2 and above - return values from this number upwards.
		271-274		Interpretation of soft keys (alone, with Shift, with Ctrl and with both). Meaning as for previous
		275		Escape key status (0 - escape enabled, non-0 - key gives ASCII value)
		276		Escape side-effects status (0 - enabled, non-0 - disabled). See documentation of OSBYTE &E6 for what the effects are.
		277-279		IRQ bit masks for user 6522, 6850, system 6522
		27A		Tube presence flag (0 - no tube, FF - tube present)
		27B		Reserved for speech system (Speech system presence flag)
		27C		Character destination status (as set by *FX 3)
		27D		Cursor editing status (as set by *FX 4)
		27E		Numeric keypad interpretation base number (This is the value returned by keypad 0. Other keypad keys return values fixed relative to this.)
		27F		*SHADOW state (0 - shadow on, non-0 - as specified by entered mode number)
		280		Country flag (0 - UK; 1 - USA)
		281		User flag (as used by *FX 1)
		282		RAM copy of serial ULA register
		283		Timer switch state (swapped between 5 and A)
		284		Soft key consistency flag
		285		Printer type (as set by *FX 5)
		286		Printer ignore character
		287-289		Break intercept vector
		28A		Memory written to by VDU driver (shadow or main)
		28B		Memory displayed (shadow or main)
		28C		Sideways memory slot of current language
		28D		Last Break type (0 - soft, 1 - power on, 2 - hard)
		28E		Effect of Shift on numeric keypad (0 - enabled, non-0 - disabled)
		28F		Startup options
290 - 2FF		Miscellaneous MOS variables, etc.:
		290,291		*TV values
		292-296		Main timer value (1st copy)
		297-29B		Main timer value (2nd copy)
		29C-2A0		Interval timer value
		2A1-2B0		Paged Sideways memory type table
		2B1,2B2		Inkey countdown counter
		2B3-2B5		OSWORD 0 workspace
		2B6-2B9		Last ADC values (LSBs)
		2BA-2BD		Last ADC values (MSBs)
		2BE		Last ADC channel to finish conversion
		2BF-2C8		Event enable flags (as set by *FX 14)
		2C9		Auto-repeat count for next key
		2CA-2CC		Two-key rollover workspace
		2CD		Sound semaphore (0 means that an envelope is being processed)
		2CE-2D6		Buffer busy flags (bit 7 set if corresponding buffer empty)
		2D7-2DF		Buffer removal pointers
		2E0-2E8		Buffer insertion pointers
		2E9,2EA		Block size of CFS open input file
		2EB		Block flag for current CFS open input file
		2EC		Last character of current block of CFS open input file
		2ED		Unused
		2EE-2FF		OSFILE control block for *LOAD and *SAVE
300 - 37F		VDU variables:
		300-307		Graphics window (internal co-ordinates)
		308-30B		Text window
		30C-30F		Graphics origin (external co-ordinates)
		310-313		Graphics cursor position (external co-ordinates)
		314-317		Previous graphics cursor (internal co-ordinates)
		318,319		Text cursor position
		31A		Line within current graphics character of current graphics point
		31B-323		VDU queue
		324-327		Graphics cursor position (internal co-ordinates)
		328-349		General graphics co-ordinate workspace
		34A,34B		Address of text cursor (as seen by 6845)
		34C,34D		Number of bytes in character row of text window
		34E		Address of bottom of screen memory (MSB)
		34F		Number of bytes used by a single character
		350,351		Address of top left of screen display
		352,353		Number of bytes in character row of whole screen
		354		Size of screen memory (MSB)
		355		Current screen mode (0 to 7 only – no "shadow" component)
		356		Screen memory map type (0 - 20kb modes; 1 - 16kb; 2 - 10kb; 3 - 8kb; 4 - teletext)
		357,358		Foreground and background text colour masks
		359		Foreground/background flag for graphics plotting (0 - foreground; 8 - background)
		35A		Current graphics plot mode
		35B		Current graphics foreground plot mode
		35C		Current graphics background plot mode
		35D,35E		Jump vector containing address of routine for processing current VDU sequence
		35F		Setting of 6845 cursor start register
		360		Number of logical colours (less 1)
		361		Pixels per byte (less 1)
		362,363		Masks for leftmost and rightmost pixels in a byte
		364,365		Output cursor position when cursor editing
		366		Cursor control flags (as set by VDU 23,16)
		367		Dot pattern for dotted lines
		368		Current state of dot pattern
		369-36B		Flags for solid/ECF ("Extended Colour Fill") plotting
		36C		"Column 81" flag
		36D,36E		Graphics foreground and background colours
		36F-37E		RAM copy of current palette
		37F		Reserved
380 - 3DF		Cassette filing system workspace
3E0 - 3FF		Keyboard input buffer
400 - 7FF		Current language main workspace, or principal Tube host code if the Tube is active
800 - 87F		Sound processing workspace
880 - 8BF		Printer buffer
8C0 - 8FF		Storage space for envelopes 1 to 4
900 - 9BF		Storage space for envelopes 5 to 16      or First part of cassette output buffer      or RS423 output buffer
9C0 - 9FF		Rest of cassette output buffer
A00 - AFF		Cassette input buffer      or RS423 input buffer
B00 - CFF		Econet workspace
D00 - D5F		NMI main routine
D60 - D7F		Econet workspace
D80 - D91		Unused
D92 - D9E		Workspace for Mouse (or Trackerball)
D9F - DEF		Extended vectors
DF0 - DFF		Paged sideways ROM/RAM workspace. One byte each (usually pointing to private workspace)
E00 - 7FFF		Main memory

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