99 lines
2.1 KiB
ArmAsm
99 lines
2.1 KiB
ArmAsm
.setcpu "65816"
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.autoimport on
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.importzp sp
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.export _gsram_getsize
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.macro A8
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sep #$20 ; put the 65C816 in 8-bit accumulator mode
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.A8
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.endmacro
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.macro I8
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sep #$10 ; put the 65C816 in 8-bit index register mode
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.I8
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.endmacro
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.macro AI8
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sep #$30 ; put the 65C816 in 8-bit accumulator and index register mode
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.A8
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.I8
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.endmacro
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.macro A16
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rep #$20 ; put the 65C816 in 8-bit accumulator mode
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.A16
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.endmacro
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.macro I16
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rep #$10 ; put the 65C816 in 8-bit index register mode
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.I16
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.endmacro
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.macro AI16
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rep #$30 ; put the 65C816 in 8-bit accumulator and index register mode
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.A16
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.I16
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.endmacro
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.code
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.proc _gsram_getsize: near
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.A8
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.I8
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; Preamble
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php ; Push status
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sei ; Disable interrupts
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clc ; Clear carry
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xce ; Clear emulation bit
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php ; Push status
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phb ; Push bank
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AI8
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; Store bank number at address 0xFFFF in each bank
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ldy #$7F ; Start at bank 0x7F
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BankSetLoop:
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phy ; Push future bank
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plb ; Pull bank
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lda $8000 ; Get address 0xFFFF in this bank
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pha ; Save old address 0xFFFF contents
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tya ; A = Y
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eor #$FF ; Flip all bits
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tay ; Y = A
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sty $8000 ; Overwrite address 0xFFFF with bank number
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eor #$FF ; Flip all bits back
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tay ; Y = A
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dey ; Decrement bank number
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cpy #$FF ; Have we wrapped around?
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bne BankSetLoop ; If not, repeat
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; Count banks with matching bank number
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ldy #$00 ; Y is bank
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ldx #$00 ; X is count
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CountLoop:
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phy ; Push future bank
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plb ; Pull bank
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tya ; A = Y
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eor #$FF ; Flip all bits
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tay ; Y = A
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cpy $8000 ; Is bank num stored at address 0xFFFF?
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bne AfterInc ; If not, skip increment
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inx ; If so, increment bank count
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AfterInc:
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eor #$FF ; Flip all bits back
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tay ; Y = A
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pla ; Get contents to restore
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sta $8000 ; Restore address 0xFFFF in this bank
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iny ; Move to next bank
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cpy #$80 ; Are we at bank 0x80 yet?
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bne CountLoop ; If not, repeat
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; Postamble
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plb ; Restore bank
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plp ; Restore status
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xce ; Restore emulation bit
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txa ; Transfer bank count to A
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plp ; Pull status again to pull I flag
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rts
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.endproc
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