Commodore 64 EasyFlash RAM driver [CA65 assembler]

driver.s

;------------------------------------------------------------------------------
;    [ efram chunk ]
;         I wrote this because I needed something that could bank in/out the
;         EasyFlash code, perform copies and memory swaps, and not occupy RAM
;         where the game exists, all while fitting in the lower half of the
;         page (because EasyAPI uses the upper half.)
;         This starts at $DF03 to give us XBANK address at $DF00 and two
;         variables to do whatever with at $DF01 and $DF02. Plus it fits nicely
;         if you think of this as a jumptable entry address or something.
;         (Don't mind my rants, these are notes to help ME think.)
;------------------------------------------------------------------------------

.scope    EFRAM
BASE = $DF03-START

;------------------------------------------------------------------------------
;    [ rom call jump ]
;         A = low address to jump. If high address is needed too, write it
;         externally or something. I use $A000 because it fits my driver.
;------------------------------------------------------------------------------

START:
          sta ROMADDR+1+BASE
          jsr ENABLE+BASE
ROMADDR:  jsr $A000

;------------------------------------------------------------------------------
;    [ disable ef-rom ]
;         Turn off the EasyFlash LED and banking.
;         The "bne" instruction is used at the end to save space.
;------------------------------------------------------------------------------

DISABLE:
ROMCONF:  lda #$F5
          pha
          lda #%00000100
          bne SWCOMMON

;------------------------------------------------------------------------------
;    [ swap ]
;         note: source and destination must be written externally.
;         Use SWAPSRC1, SWAPSRC2, SWAPDST1, and SWAPDST2.
;         $FFFF is used as a temp address because it's all one bits and this
;         somehow benefits the life of the EasyFlash.
;         This only does up to one page at a time (the length is determined by
;         the X register) and banks out EasyFlash during the operation. It will
;         bank EasyFlash back in after it is finished.
;         Also note that due to the way the indexing is done, the swapping will
;         start at ADDRESS+1. Not a problem if X is zero, it will do a whole
;         page. But keep this in mind for all other values.
;------------------------------------------------------------------------------

SWAP:
.scope    SWAPSEC
          jsr DISABLE+BASE
LOOP:
SRC1:     lda $FFFF,x
DST1:     ldy $FFFF,x
DST2:     sta $FFFF,x
          tya
SRC2:     sta $FFFF,x
          dex
          bne LOOP
.endscope

;------------------------------------------------------------------------------
;    [ enable ef-rom ]
;         All other roms are enabled as a side-effect & EasyFlash LED is on.
;------------------------------------------------------------------------------

ENABLE:
          lda $01
          sta ROMCONF+1+BASE
          ora #$07
          pha
          lda #%10000111

;------------------------------------------------------------------------------
;    [ set ef-rom state ]
;         A = EasyFlash $DE02 state
;------------------------------------------------------------------------------
          
SWCOMMON:
          sta $DE02
          pla
          sta $01

;------------------------------------------------------------------------------
;    [ retore bank ]
;         This is the ONLY return point from the loader.
;         All functions lead here. Functions may also JSR here just to set
;         the EasyFlash bank.
;------------------------------------------------------------------------------

SETBANK:
BNK:      lda #$01
RESTBNK:
          sta $DE00
          rts

;------------------------------------------------------------------------------
;    [ copy ]
;         note: source and destination must be written externally to
;         SRC+1/SRC+2 and DST+1/DST+2.
;         X = highbyte length (two's complement)
;         Y = lowbyte length (two's complement)
;         The "bne RESTBNK" line is the *actual* exit point in this function.
;         I have tried a number of methods and this hybrid self-modifying and
;         absolute-indexing method is the fastest I could get. It does require
;         a bit of setup due to the file length needing to be two's complement.
;         Even using zeropage was slower because of the indirect addressing.
;         BIT+BVC is used to determine if we are crossing EasyFlash banks.
;         Due to this, you CANNOT tell it not to use the high bank. However,
;         by writing different values to COPYWRP you can at least tell it you
;         don't want the low bank (the value $A0 will work OK for this.) The
;         default wrap value is $80.
;------------------------------------------------------------------------------

COPY:
.scope    COPYSEC
          jsr SETBANK+BASE
LOOP:
SRC:      lda $FFFF,y
DST:      sta $FFFF,y
          iny
          beq BANKCHK
CTROK:
          inx
          bne LOOP
          inc LEN+BASE
          bne LOOP
          lda #$01
          sta BNK+1+BASE
          bne RESTBNK
BANKCHK:          
          inc SRC+2+BASE
          inc DST+2+BASE
          bit SRC+2+BASE
          bvc CTROK
WRP:      lda #$80
          sta SRC+2+BASE
          inc BNK+1+BASE
          jsr SETBANK+BASE
          jmp CTROK+BASE
LEN:      .byte $00
.endscope

;------------------------------------------------------------------------------
;    [ footer ]
;         I used to put padding here, but now that this routine is pretty much
;         finished I removed it.
;------------------------------------------------------------------------------

END:
.endscope
EFRAMSIZE = EFRAM::END-EFRAM::START

;------------------------------------------------------------------------------
;    [ exports ]
;         To be used when writing internal variables externally.
;------------------------------------------------------------------------------

EFMEMCONF := EFRAM::ROMCONF+1+EFRAM::BASE
EFDISABLE := EFRAM::DISABLE+EFRAM::BASE
EFENABLE  := EFRAM::ENABLE+EFRAM::BASE
EFCOPY    := EFRAM::COPY+EFRAM::BASE
COPYSRC   := EFRAM::COPYSEC::SRC+1+EFRAM::BASE
COPYDST   := EFRAM::COPYSEC::DST+1+EFRAM::BASE
COPYBNK   := EFRAM::BNK+1+EFRAM::BASE
COPYWRP   := EFRAM::COPYSEC::WRP+1+EFRAM::BASE
COPYLEN   := EFRAM::COPYSEC::LEN+EFRAM::BASE
EFSWAP    := EFRAM::SWAP+EFRAM::BASE
SWAPSRC1  := EFRAM::SWAPSEC::SRC1+1+EFRAM::BASE
SWAPSRC2  := EFRAM::SWAPSEC::SRC2+1+EFRAM::BASE
SWAPDST1  := EFRAM::SWAPSEC::DST1+1+EFRAM::BASE
SWAPDST2  := EFRAM::SWAPSEC::DST2+1+EFRAM::BASE

See this driver in action on the Legacy of the Ancients / Legend of Blacksilver +4 release on CSDB:
http://csdb.dk/release/?id=128480