75 lines
1.7 KiB
ArmAsm
75 lines
1.7 KiB
ArmAsm
FR0 = $d4 ; float48
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PORTB = $d301
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EXTENDED_RAM = $4000 ; 16KiB bank on the XE
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; lookup table for top byte -> PORTB value for bank-switch
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.align 256
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bankswitch:
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.repeat 256, i
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.byte ((i & $c0) >> 5) | $c1
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.endrepeat
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; 58-77 cycles
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; clobbers x, y, dest to dest + 3
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.macro imul8xe dest, arg1, arg2
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.local done
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.local output
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.local ptr
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output = dest
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ptr = dest + 2 ; scratch space assumed
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; bottom 14 bits except the LSB are the per-bank table index
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; add $4000 for the bank pointer
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lda arg1 ; 3 cyc
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and #$fe ; 2 cyc
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sta ptr ; 3 cyc
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lda arg2 ; 3 cyc
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and #$3f ; 2 cyc
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clc ; 2 cyc
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adc #$40 ; 2 cyc
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sta ptr + 1 ; 3 cyc
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; top 2 bits are the table bank selector
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ldx arg2 ; 3 cyc
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lda bank_switch,x ; 4 cyc
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sta PORTB ; 4 cyc
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; copy the entry into output
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ldy #0 ; 2 cyc
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lda (ptr),y ; 5 cyc
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sta output ; 3 cyc
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iny ; 2 cyc
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lda (ptr),y ; 5 cyc
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sta output+1 ; 3 cyc
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; note: we are not restoring memory to save 6 cycles!
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; this means those 16kb have to be switched back to base RAM
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; if we need to use them anywhere else
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;;; restore memory
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;;lda #$81 ; 2 cyc - disabled
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;;sta PORTB ; 4 cyc - disabled
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; check that 1 bit we skipped to fit into space
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lda arg1 ; 3 cyc
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and #1 ; 2 cyc
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beq done ; 2 cyc
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; add the second param one last time for the skipped bit
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clc ; 2 cyc
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lda arg2 ; 3 cyc
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adc output ; 3 cyc
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sta output ; 3 cyc
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lda #0 ; 2 cyc
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adc output+1 ; 3 cyc
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sta output+1 ; 3 cyc
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done:
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.endmacro
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proc imul8xe_init
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rts
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endproc
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