Applecorn/applecorn.s

* APPLECORN.S
* (c) Bobbi 2021 GPLv3
*
* Load an Acorn BBC Micro ROM in aux memory and
* Provide an environment where it can run
*
* Assembled with the Merlin 8 v2.58 assembler on Apple II.

            XC                ; 65c02
            ORG   $2000       ; Load addr of loader in main memory

* Monitor routines
BELL        EQU   $FBDD
PRBYTE      EQU   $FDDA
COUT1       EQU   $FDED
CROUT       EQU   $FD8E
HOME        EQU   $FC58
AUXMOVE     EQU   $C311
XFER        EQU   $C314

* Monitor ZP locations
A1L         EQU   $3C
A1H         EQU   $3D
A2L         EQU   $3E
A2H         EQU   $3F
A4L         EQU   $42
A4H         EQU   $43

* Used by XFER
STRTL       EQU   $3ED
STRTH       EQU   $3EE

* Reset vector (2 bytes + 1 byte checksum)
RSTV        EQU   $3F2

* IRQ vector
A2IRQV      EQU   $3FE

* MLI entry point
MLI         EQU   $BF00

* IO Buffer for reading file (1024 bytes)
IOBUF0      EQU   $0C00       ; For loading/saving, OSFILE, *.
IOBUF1      EQU   $1000       ; Four open files for langs
IOBUF2      EQU   $1400
IOBUF3      EQU   $1800
IOBUF4      EQU   $1C00

* 512 byte buffer sufficient for one disk block
BLKBUF      EQU   $5000       ; Can't use $400 as ProDOS uses
BLKBUFEND   EQU   $5200       ;  'hidden' bytes within screen

* Address in aux memory where ROM will be loaded
AUXADDR     EQU   $8000

* Address in aux memory where the MOS shim is located
AUXMOS1     EQU   $2000       ; Temp staging area in Aux
EAUXMOS1    EQU   $5000       ; End of staging area
AUXMOS      EQU   $D000       ; Final location in aux LC

* Called by code running in main mem to invoke a
* routine in aux memory
XF2AUX      MAC
            LDX   $C08B       ; R/W LC RAM, bank 1
            LDX   $C08B
            LDX   #<]1
            STX   STRTL
            LDX   #>]1
            STX   STRTH
            SEI               ; Disable IRQ before XFER
            SEC               ; Use aux memory
            BIT   $FF58       ; Set V: use alt ZP and LC
            JMP   XFER
            EOM

* Called by code running in aux mem to invoke a
* routine in main memory
XF2MAIN     MAC
            LDX   STRTL
            STX   STRTBCKL
            LDX   STRTH
            STX   STRTBCKH
            LDX   #<]1
            STX   STRTL
            LDX   #>]1
            STX   STRTH
            SEI               ; Disable IRQ before XFER
            TSX
            STX   $0101       ; Save alt SP
            LDX   $0100       ; Load main SP into X
            CLC               ; Use main mem
            CLV               ; Use main ZP and LC
            JMP   XFER
            EOM

* Macro called on re-entry to aux memory
* Careful: This enables IRQ - not for use in ISR
ENTAUX      MAC
            LDX   $0101       ; Recover alt SP
            TXS
            CLI               ; Re-enable IRQ after XFER
            LDX   STRTBCKL
            STX   STRTL
            LDX   STRTBCKH
            STX   STRTH
            EOM

* Macro called on re-entry to main memory
* Careful: This enables IRQ - not for use in ISR
ENTMAIN     MAC
            TXS               ; Main SP already in X
            LDX   $C081       ; Bank in ROM
            LDX   $C081
            CLI               ; Re-enable IRQ after XFER
            EOM

* Macro called on re-entry to aux memory
* For use in interrupt handlers (no CLI!)
IENTAUX     MAC
            LDX   $0101       ; Recover alt SP
            TXS
            LDX   STRTBCKL
            STX   STRTL
            LDX   STRTBCKH
            STX   STRTH
            EOM

* Macro called on re-entry to main memory
* For use in interrupt handlers (no CLI!)
IENTMAIN    MAC
            TXS               ; Main SP already in X
            LDX   $C081       ; Bank in ROM
            LDX   $C081
            EOM

* Enable writing to main memory (for code running in aux)
WRTMAIN     MAC
            SEI               ; Keeps IRQ handler easy
            STA   $C004       ; Write to main memory
            EOM

* Go back to writing to aux (for code running in aux)
WRTAUX      MAC
            STA   $C005       ; Write to aux memory
            CLI               ; Normal service resumed
            EOM

* Manually enable AltZP (for code running in main)
ALTZP       MAC
            SEI               ; Disable IRQ when AltZP on
            LDA   $C08B       ; R/W LC bank 1
            LDA   $C08B
            STA   $C009       ; Alt ZP and LC
            EOM

* Manually disable AltZP (for code running in main)
MAINZP      MAC
            STA   $C008       ; Main ZP and LC
            LDA   $C081       ; Bank ROM back in
            LDA   $C081
            CLI               ; Turn IRQ back on
            EOM

* Code is all included from PUT files below ...
* ... order matters!
            PUT   LOADER
            PUT   MAIN.ROMMENU
            PUT   MAINMEM
            PUT   AUXMEM.MOSEQU
            PUT   AUXMEM.INIT
            PUT   AUXMEM.VDU
            PUT   AUXMEM.HOSTFS
            PUT   AUXMEM.OSCLI
            PUT   AUXMEM.BYTWRD
            PUT   AUXMEM.CHARIO
            PUT   AUXMEM.MISC

* Automatically save the object file:
            SAV   APPLECORN