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Table of Contents
The monitor ROM reserves 256 bytes from addresses 2000 to 20FF (on MO machines) for its own use. If you want to erase this area, you must keep interrupt disabled, and you won't be able to use SWI calls anymore.
Textmode screen
00-18: TERMIN - Logical line ends
This allows the BASIC editor to work with lines longer than 40 characters. For each of the 24 lines on screen, its actual length (which may be more than 40 chars) is stored. When you press the ENT key, the line under the cursor is scanned using this length information. When using the DEL and INS keys, this is used to know how much characters to shift.
19: STATUS - I/O bits
- bit 0: Keyboard scan is up to date
- bit 1: keyboard repeat is enabled
- bit 2: Cursor is visible
- bit 3: keyboard beep is disabled
- bit 4: text drawing is in transparent mode
- bit 6: smooth scrolling is active
- bit 7: caps lock
1A: TABPT - Tabulation pointer
Points to the current line terminator (?)
1B: RANG - Cursor X position
From 0 to 39.
1C: COLN - Cursor Y position
From 0 to 24.
1D: TOPTAB - Window top position
1E: TOPRAN - Window left position
1F: BOTTAB - Window bottom position
20: BOTRAN - Window right position
21-22: SCRPT - Screen pointer
23-24: STADR - Window start address
25-26: ENDDR - Window end address
27-28: BLOCZ - Always 0
This value is always 0. This is apparently used when clearing other stuff, in order to not need an extra CPU register to store a 0.
29: FORME - Character color
Select the color used when drawing.
This can be a value from -16 to 15. If positive, the color will be set as the character color for the affected cell, and the pixel will be set to 1. If negative, the color will be set as the background color, and the pixel set to 0.
2A: ATRANG - Attribute
Defines the currently active text attributes:
- Bit 0: double width
- Bit 1: double height
- Bit 6: foreground color change for whole screen
- Bit 7: background color chnage for whole screen
2B: COLOR
Store foreground and background colors in the same format as the attribute RAM. Used by PUTCH.
2C: PAGFLG - Page mode flag
Setting this to 1 disables screen scrolling and uses page mode (printing after line 25 goes back to line 0).
2D: SCROLS - Smooth scrolling
Setting this to 255 enables smooth pixel-scrolling.
2E: CURSFL - Cursor end-of-line marker
Remembers that the cursor is on the last screen-line of a logical line. Do not link that logical line with the next one when the cursor is moved away.
2F: COPCHR - Copy character
Holds ASCII code of the character under the cursor.
30: EFMCPT - Cursor blink counter
Decremented by interrupts, when it reaches 0, the cursor state is changed.
31: ITCMPT - Interrupt counter
General looping interrupt counter. Incremented by 50Hz interrupt.
Graphics
32-33, 34-35: PLOTX, PLOTY - Drawing position
Set by all drawing routines to the current pen position. Used when drawing lines as the 'from' point.
36: CHDRAW - Character draw
Used as the character to use for drawing routines:
- 0: do pixel drawing
- Other: ASCII code of character to use
Keyboard
37: KEY
Set by GETCH to the pressed key keycode.
38: CMPTKB - Keyboard repeat delay counter
Sound
3A: TEMPO
Note length multiplier
3C: DUREE
Note length
3D: WAVE
Sound wave shape (values from 0 to 5)
3E: OCTAVE
Note octave multiplier (16, 8, 4, 2, 1, from bass to treble)
Tape drive
Note: this needs a tape drive.
40: K7DATA
Byte being read or written to tape
41: K7LENG
Byte counter for current block being written to tape
Printer
Note: this needs a printer port, which is an extension on some machines.
42: PROPC - Printer Operation Code
43: PRSTA - Printer status
- 4: port open
- 8: ready
- 10: port closed
System support
44-45: TEMP - Temporary register
Used as temporary storage by various routines.
46-47: SAVEST - System stack pointer backup
S register is saved here when a routine needs to modify it.
Disk operations
These variables are only used by the floppy controller ROM, not the monitor one.
48: DKOPC - Disk Operation
Used by DKCONT system call. Possible operations:
- 1: initialize controller
- 2: read a sector
- 4: switch to single density
- 8: write a sector
- 10: switch to double density
- 20: seek track 0
- 40: seek track
- 80: verify sector.
Some of these can be combined (88 is “write and verify sector”).
49: DKDRV - Active drive
- 0: Drive A, top side
- 1: Drive A, bottom side
- 2: Drive B, top side
- 3: Drive B, bottom side
- 4: RAM disk
4A-4B: DKTRK - Disk track
This is a 16 bit variable because of the use on QDD as the “LBA” block number.
4C: DKSEC - Disk sector number
4D: DKNUM - Sector interleaving factor
The default is 7, which lays out the sectors in this order: 0,7,14,6,13,5,12,4,11,3,10,2,9,1,8
4E: DKSTA - Disk status
Used as error bits after executing disk operations:
- bit 0: write protection
- bit 1: track error
- bit 2: sector error
- bit 3: data error
- bit 4: drive not ready, no disk or motor off
- bit 5: compare error (data read from disk and memory version are different)
- bit 6: not formatted
Used as information about the type of controller aftre initialisation:
- 43 ('C'): single density controller
- 44 ('D'): double density controller
4F-50: DKBUF - Disk buffer
Pointer to 256 byte work buffer for the floppy controller
51-58: working area
Used for several different purposes by the floppy ROM.