* = $FE9D

; Fresh implementation of the +btxfer.fe48 code found on CBUG disk 2
; created by splicing together the actual dumped kernal ROM image 
; from 901244-04a and the commented source code available at
; http://www.6502.org/users/sjgray/computer/cbm2/transactor-b-xfer.pdf
;
; Note if you're running this in an emulator that IPINIT assumes
; it's going to be reading $FF from $01FF and fails if it doesn't.
; The same goes for the original CBUG +btxfer.fe48.
; 
; This code should load at fe9d
; 
; 2020 Dave McMurtrie

IPOINT   = $00AC
E6509    = $00
I6509    = $01
STACKP   = $01FF

EXSUBF   PHP          ;save status
         PHA          ;save .a
         LDA #$0F     ;system segment
         STA I6509    ;set up ind pointer
         PLA          ;restore .a
         PLP          ;restore status
         JMP EXSUB
         NOP 
         NOP 
         NOP 
         NOP
         NOP
         NOP
         NOP
         NOP
         NOP
         NOP
EXSUB    PHP            ;save status
         SEI
         PHA            ;.a
         TXA
         PHA            ;.x
         TYA
         PHA            ;.y
         JSR IPINIT     ;init ipoint, stack
         TAY            ;.y is xfer sp
         LDA E6509      ;return bank
         JSR PUTAS     ;push .a other stack
         LDA #$04       ;xfer seg...
         LDX #$FF       ;...rts routn
         JSR PUTAXS     ;.a.x to other stack
         TSX
         LDA $0105,X    ;.sp + 5 adds lo
         SEC
         SBC #$03       ;-3 for jsr
         PHA            ;save .a
         LDA $0106,X    ;hi addr
         SBC #$00
         TAX            ;.x hi
         PLA            ;restore .a lo
         JSR PUTAXS     ;push .a.x
         TYA            ;xfer seg sp
EXCOMM   SEC
         SBC #$04       ;4 bytes .y.x.a.p
         STA STACKP     ;xfer seg new sp
         TAY            ;new sp also
         LDX #$04       ;4 bytes .y.x.a.p
EXSU10   PLA
         INY
         STA (IPOINT),Y ;regs other stack
         DEX
         BNE EXSU10
         LDY STACKP     ;restore .y as sp
         LDA #$2D       ;routine to
         LDX #$FF       ;...pull regs
         JSR PUTAXS    ;...to other stack
         PLA
         PLA
EXGBYE   TSX
         STX STACKP     ;save sp
         TYA            ;.y is new sp
         TAX
         TXS            ;new stack pointer
         LDA I6509      ;xfer seg#
         JMP GBYE       ;good bye
         NOP
EXCRTS   PHP            ;.p
         PHP            ;.p
         SEI
         PHA            ;.a
         TXA
         PHA            ;.x
         TYA
         PHA            ;.y
         TSX
         LDA $0106,X    ;.sp + 6 is...
         STA I6509      ;...return seg
         JSR IPINIT     ;load cross sp
         JMP EXCOMM
IPINIT   LDY #$01
         STY IPOINT+1
         DEY
         STY IPOINT
         DEY
         LDA (IPOINT),Y
         RTS
PUTAXS   PHA               ;save .a
         TXA
         STA (IPOINT),Y    ;.x hi
         DEY
         PLA
PUTAS    STA (IPOINT),Y    ;.a lo
         DEY
         RTS
EXPULL   PLA
         TAY               ;.y
         PLA
         TAX               ;.x
         PLA               ;.a
         PLP               ;.p
         RTS
         PHP
         JMP ($FFFA)
         BRK
         NOP
         RTS
         CLI 
         RTS 
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX
         TAX 
         TAX
         TAX
         TAX
         TAX 
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF 
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF 
         JSR EXSUBF 
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
         JSR EXSUBF
GBYE     STA E6509      ;goodbye...
         RTS
         BRK
         BRK
         BRK
         BRK
         BRK
         BRK