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.
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 change for whole screen
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.
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
Set by GETCH to the pressed key keycode.
38: CMPTKB - Keyboard repeat delay counter
Note length multiplier
Sound wave shape (values from 0 to 5)
Note octave multiplier (16, 8, 4, 2, 1, from bass to treble)
Note: this needs a tape drive.
Byte being read or written to tape
Byte counter for current block being written to tape
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
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.
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 after 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.
Keyboard scanning, interrupts, custom character tables
59: SEQUCE - Screen rendering state
This is used as state saving for the screen display state machine. In particular, it allows rendering of multi-byte accented characters.
Temporary parameter storage for US escape sequences
Temporary storage for multi-byte accented characters
5C,5D: SS2GET, SS3GET
Screen State machine temporary storage. These store the two parameters for multi-byte accent sequences.
5E-5F: SWIPT - Software interrupt table pointer
This points to the table of system calls used by the SWI handler. Modifying this value allows redirecting the handlers for your own use.
61-62: TIMEPT - Ticker routine pointer
This points to a routine to be called on every interrupt.
Set to a non-zero value to re-route the IRQ handler using the IRQPT below
Pointer to a routine to call on interrupt when SEMIRQ is set.
Pointer to a routine to call on FIRQ. This is either the lightpen scan, or the barcode reader.
Pointer to keyboard decoding table (used by GETCH)
Pointer to user characters for codes 128-255.
Pointer to characters for codes 32-127. Characters are stored bottom to top.
76: LATCLV - Keyboard repeat delay
Control char to send to printer to switch to graphic mode.
Lightpen calibration offset
Default density for current floppy controller.
Set to FF if there is a floppy controller connected.
82-85: serial port variables
These are not used for anything in the ROM, but the software provided with the RF57 serial extension uses this area.
81-CC: system stack
The stack grows downwards.
Working area for the lightpen reading code.
FE-FF: FSTRST - Fast Reset
This is set on first boot after the direct-page has been initialized. The Reset handler checks for this, and doesn't repeat all initializations (memory clearing, etc) if this is the case.