mandel.s

@@ -74,6 +74,9 @@ width = 160
 half_width = width >> 1
 stride = width >> 2
 
+EXTENDED_RAM = $4000 ; 16KiB bank on the XE
+PORTB  = $D301 ; memory & bank-switch for XL/XE
+
 DMACTL = $D400
 DLISTL = $D402
 DLISTH = $D403
@@ -428,6 +431,176 @@ fill_masks:
     .endscope
 .endmacro
 
+; lookup table for top byte -> PORTB value for bank-switch
+;.align 256 ; warning - if not aligned this will cost an extra cycle sometimes
+bankswitch:
+    .repeat 256, i
+        .byte ((i & $c0) >> 5) | $c1
+    .endrepeat
+
+; 58-77 cycles
+; clobbers x, y, dest to dest + 3
+.macro imul8xe dest, arg1, arg2
+.local done
+.local output
+.local ptr
+
+    output = dest
+    ptr = dest + 2 ; scratch space assumed
+
+    ; bottom 14 bits except the LSB are the per-bank table index
+    ; add $4000 for the bank pointer
+    lda arg1     ; 3 cyc
+    and #$fe     ; 2 cyc
+    sta ptr      ; 3 cyc
+    lda arg2     ; 3 cyc
+    and #$3f     ; 2 cyc
+    clc          ; 2 cyc
+    adc #$40     ; 2 cyc
+    sta ptr + 1  ; 3 cyc
+    
+    ; top 2 bits are the table bank selector
+    ldx arg2          ; 3 cyc
+    lda bank_switch,x ; 4 cyc
+    sta PORTB         ; 4 cyc
+
+
+    ; copy the entry into output
+    ldy #0       ; 2 cyc
+    lda (ptr),y  ; 5 cyc
+    sta output   ; 3 cyc
+    iny          ; 2 cyc
+    lda (ptr),y  ; 5 cyc
+    sta output+1 ; 3 cyc
+
+    ; note: we are not restoring memory to save 6 cycles!
+    ; this means those 16kb have to be switched back to base RAM
+    ; if we need to use them anywhere else
+    ;;; restore memory
+    ;;lda #$81     ; 2 cyc - disabled
+    ;;sta PORTB    ; 4 cyc - disabled
+
+    ; check that 1 bit we skipped to fit into space
+    lda arg1     ; 3 cyc
+    and #1       ; 2 cyc
+    beq done     ; 2 cyc
+
+    ; add the second param one last time for the skipped bit
+    clc          ; 2 cyc
+    lda arg2     ; 3 cyc
+    adc output   ; 3 cyc
+    sta output   ; 3 cyc
+    lda #0       ; 2 cyc
+    adc output+1 ; 3 cyc
+    sta output+1 ; 3 cyc
+
+done:
+.endmacro
+
+.macro bank_switch bank
+    lda #((bank << 1) | $c1)
+    sta PORTB
+.endmacro
+
+.proc imul8xe_init
+
+    ; go through the input set, in four 16KB chunks
+
+    arg1 = FR1
+    arg2 = FR2
+    result = FR0
+
+    lda #$00
+    sta arg1
+    sta arg2
+
+    ; $00 * $00 -> $3f * $ff
+    bank_switch 0
+    jsr imul8xe_init_section
+
+    ; $40 * $00 -> $7f * $ff
+    bank_switch 1
+    jsr imul8xe_init_section
+
+    ; $80 * $00 -> $bf * $ff
+    bank_switch 2
+    jsr imul8xe_init_section
+
+    ; $c0 * $00 -> $ff * $ff
+    bank_switch 3
+    jsr imul8xe_init_section
+
+    rts
+.endproc
+
+; Initialize a 16 KB chunk of the table
+; input: multipliers in temp
+; output: new multipliers in temp
+; clobbers: temp, temp2
+.proc imul8xe_init_section
+    arg1 = FR1
+    arg2 = FR2
+    result = FR0
+    ptr = temp2
+
+    lda #$00
+    sta ptr
+    lda #$40
+    sta ptr + 1
+
+    ldx #0
+    ldy #0
+
+    ; outer loop: $00 -> $3f
+outer_loop:
+
+    ; reset result to 0
+    lda #0
+    sta result
+    sta result + 1
+
+    ; inner loop: $00 -> $ff
+inner_loop:
+
+    ; copy result to data set
+    lda result
+    sta (ptr),y
+    lda result + 1
+    sta (ptr),y
+
+    ; result += 2 * arg2
+    clc
+    lda arg2
+    adc result
+    sta result
+    lda #0
+    adc result + 1
+    sta result
+    lda arg2
+    adc result
+    sta result
+    lda #0
+    adc result + 1
+    sta result
+
+    ; inner loop check
+    inc arg1
+    inc arg1
+    inc ptr
+    inc ptr
+    bne inner_loop
+
+    ; outer loop check
+    inc arg2
+    inc ptr + 1
+    lda ptr + 1
+    cmp #$40
+    bne outer_loop
+
+    rts
+
+.endproc
+
 .proc imul16_func
     arg1 = FR0   ; 16-bit arg (clobbered)
     arg2 = FR1   ; 16-bit arg (clobbered)