Programming/Assembly
ROM autostarting:
The Plus/4 can have 8 ROMs, 4 high and 4 low. The low ROMs run from $8000 to $BFFF, and the high from $C000 to $FFFF. The ROMs can be paged out completely by using the ROM and RAM select registers located at $FF3E and $FF3F respectively. Writing to $FF3F switches out the ROMs and writing to $FF3E switches them back in. Commodore's BASIC V3.5 is able to get over 60k of memory free by switching the ROMs in and out. Basic achieves this by transferring some of its ROM down into RAM. From address $0373 to address $04E6 you'll find various routines for banking out the ROMs to read the RAM underneath. The TED chip is however not paged out.
To select one of the 8 ROMs (BASIC and the KERNALl being 2 of them), you write to locations $FDD0 to $FDDF. The lower 2 bits are used to select the low ROMs, and bits 2 and 3 are used to select the high ROMs. To select the BASIC and KERNAL ROMs (numbers 0 and 4), you write to location $FDD0. This enables ROM number 0 and 4 (both 2 bit combos are 0). To enable the 3-Plus-1 built in ROMs, you would write to location $FDD5 to enable bit pattern %0101, or ROM number 1 and 5. When you are paging ROMs, locations $FC00 to $FCFF remain unpaged.
This is where the KERNAL does its "Check 8-ROM" routine ($FC1F, KERNAL routine SPKPAT).
PAT = Physical Address Translation (PAT) table.
From memory locations $FC1E to $FCFF, are taken up with ROM banking routines.
ROMs have the following structure:
$8000 jmp cold_start
$8003 jmp warm_start
$8006 device / module number (empty = 0, autostart = 1, other = module number)
$8007 "CBM" signature
Coldstart:
If you look at the KERNAL routine at $FC1E you'll notice it checks for $43 $42 $4d at locations $8007 and if found it jumps to $8000. It's here you can set up a KEY to jump to your ROM routine.
; ** spkpat ** fixes: babbling speech module
;
spkpat
lda #$09
sta $fd20 ;shut up
ora #$80
sta $fd20 ;shut up now (texans think this way...)
jmp poll
;
;***********************************************************************
;
; software supporting banking hardware
;
; ******* this code must fit *entirely* between $fc00-$fcff *******
;
;
; set up each of the four possible slots, and test if there is a
; device in that slot. if so, store that devices number in the cor-
; responding entry in the physical address translation (pat) table.
; if a device is found to have a number of '1', it will be logged in the
; table, and a jump to that devices cold-start routine will be performed.
;
;***********************************************************************
poll
ldx #3
stx xsav
lda #0
poll10
sta pat,x ;first zero out all pat entries
dex
bpl poll10
poll20 ;set up and test each bank
ldx xsav
lda dblx,x ;set up both upper & lower banks
tax
sta bnksel,x
ldy #2
poll30
lda $8007,y ;test for 'cbm' (in ascii)
cmp cbmmsg,y
bne poll50 ;no match
dey
bpl poll30 ;keep looking
lda $8006 ;it's 'cbm'...now get device number
ldx xsav
sta pat,x
cmp #1 ;autostart?
bne poll50 ;no
stx curbnk ;yes, give them the bank configuration,
jsr $8000 ;.. then go to cold start routine.
poll50
dec xsav
bpl poll20
rts
cbmmsg .byte 'CBM'
dblx .byte %00000000,%00000101,%00001010,%00001111
1) The coldstart routine sets up a function KEY.
2) The coldstart routine sets up a function routine in RAM at $05f5-$065f.
3) The coldstart routine typically prints a message below COMMODORE BASIC V3.5 60671 BYTES FREE.
Example for 3 cartridges (without 3-Plus-1):
COMMODORE BASIC V3.5 60671 BYTES FREE
CARTRIDGE 3 ON F3
CARTRIDGE 2 ON F2
CARTRIDGE 1 ON F1
READY.
KEY
KEY 1,"SYS1525:CART 1"
KEY 2,"SYS1545:CART 2"
KEY 3,"SYS1564:CART 3"
KEY 4,"SCNCLR"+CHR$(13)
KEY 5,"DSAVE"+CHR$(34)
KEY 6,"RUN"+CHR$(13)
KEY 7,"LIST"+CHR$(13)
KEY 8,"HELP"+CHR$(13)
READY.
Warmstart:
The warmstart routine is typically the entry point to the main cartridge routine (SpeedScript, Script/Plus).
Module:
If device / module number = 0, the cartridge is ignored.
If device / module number = 1, the cartridge is autostarted.
If device / module number > 1, the cartridge is cold-started only.
Signature:
The CBM signature is mandatory so that the ROM is recognized.
ROMs are checked (after hard reset) in the following order:
0f: 00 00 11 11 (cartridge #3) External 2
0a: 00 00 10 10 (cartridge #2) External 1
05: 00 00 01 01 (cartridge #1) Internal (3-Plus-1 / 3-Plus-1)
00: 00 00 00 00 (cartridge #0) BASIC / KERNAL
ROMs are cold started in the following order (see BASIC / KERNAL is not): slots 3, 2, 1 - ext2, ext1, internal.
;*************************************************
;
; call every active cartridges cold start routine.
;
;*************************************************
phenix
sei
ldx #3
phen2
lda pat,x
beq phen3 ;no cartridge in this slot if 0
txa
pha
lda dblx,x ;select low and high rom
tax
sta bnksel,x
stx curbnk
jsr $8000 ;call it's cold start routine
pla
tax
phen3
dex
bne phen2 ;do slots 3,2,1 - ext2, ext1, int
sta bnksel ;set up system bank
stx curbnk ;..as current bank
cli
rts
Comments:
Some ROMs are badly coded and work only in slot 2 (cartridge ext1).
Some ROMs do not check for other ROMs present and blindly assign KEY 1 or 2.
Many ROMs only copy code from ROM to RAM and execute code in RAM, thus do not save precious memory (for example 3-Plus-1 copies its code from $1000 to $7000.
The 3-Plus-1 KERNAL routine copies some data into the function key area and also a start up section allowing you to run your program.
[Partially adapted from Mike Dailly].
banking,autostarting |
