micro-optimizations in imul8xe

53-72 cycles
overview in 10.896 ms/px

mandel.s

@@ -37,6 +37,7 @@ palette_offset  = $bf ; u8
 palette_ticks = $c0 ; u8
 chroma_ticks  = $c1 ; u8
 chroma_offset = $c2 ; u8
+ptr           = $c4 ; u16
 
 palette_delay = 23
 chroma_delay = 137
@@ -452,21 +453,19 @@ bank_switch_table:
 .macro imul8 dest, arg1, arg2, xe
     .if xe
         ; using 64KB lookup table
-        ; 58-77 cycles
-        ; clobbers x, y, dest to dest + 3
+        ; 53-72 cycles
+        ; clobbers x, y, dest, ptr
         .scope
             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
+            tay          ; 2 cyc
             lda arg2     ; 3 cyc
             and #$3f     ; 2 cyc
-            clc          ; 2 cyc
-            adc #$40     ; 2 cyc
+            ora #$40     ; 2 cyc
             sta ptr + 1  ; 3 cyc
             
             ; top 2 bits are the table bank selector
@@ -476,7 +475,6 @@ bank_switch_table:
 
 
             ; copy the entry into output
-            ldy #0       ; 2 cyc
             lda (ptr),y  ; 5 cyc
             sta output   ; 3 cyc
             iny          ; 2 cyc
@@ -609,6 +607,9 @@ init:
     lda #$00
     sta arg1
     sta arg2
+    sta ptr
+    lda #$40
+    sta ptr + 1
 
     ; $00 * $00 -> $3f * $ff
     bank_switch 0