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Fix up all but two of the off-by-one tick counts. This uses the

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trick of temporarily violating the X=0 invariant (which is only
required in the tick_6 opcode tail path to steal an extra cycle)
to reorder a STA $2000,Y outside of the tick loop.

The cost of this is that we don't have enough pad cycles left to JMP
to the common opcode tail, but I think this still (barely) fits in
main RAM.
This commit is contained in:
kris 2019-03-05 23:20:17 +00:00
parent 8c23824aa6
commit df25fce067
1 changed files with 49 additions and 25 deletions
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74
audiotest/audiotest/main.s
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@ -453,6 +453,8 @@ _op_tick_tail_16:
_op_tick_tail_12:
LDA WDATA ; 4
_op_tick_tail_8:
; NB: we use ,X indexing here to get an extra cycle. This requires us to
; maintain the invariant X=0 across opcode dispatch
STA _op_tick_6_jmp+1,x ; 5
_op_tick_6_jmp:
JMP op_nop ; 3
@ -495,42 +497,53 @@ op_tick_14:
JMP _op_tick_tail_52 ; 3+52
; 4+(4+4+4+4)+5+2+3+43
op_tick_16:
BIT tick ; 4
LDA WDATA ; 4
; This lets us share a common opcode tail; otherwise we need a STA dummy 4-cycle opcode
; which doesn't leave enough to JMP with.
; This temporarily violates X=0 invariant required by tick_6
LDX WDATA ; 4
LDY WDATA ; 4
STA dummy ; 4
BIT tick ; 4
STA $2000,x ; 5
LDX #$00 ; 2 restore X=0 invariant
JMP _op_tick_tail_43 ; 3 + 43
; 4 + (4+4+4+2+4)+5+5+2+2+4+5+4+5+4+4+4+4+3
op_tick_18:
BIT tick ; 4
LDA WDATA ; 4
LDY WDATA ; 4
; lets us reorder the 5-cycle STA $2000,y outside of tick loop.
; This temporarily violates X=0 invariant required by tick_6
LDX WDATA ; 4
NOP ; 2
BIT tick ; 4
STA $2000,Y ; 5
STA $2000,X ; 5
LDX #$00 ; 2 restore X=0 invariant
; used >3 pad cycles within tick loop; can't branch to tail
; XXX can use LDX WDATA instead of STA dummy? only if we no longer need X=0 invariant
NOP ; 2
STA $2000,Y ; 5
LDY WDATA ; 4
STA $2000,Y ; 5
LDY WDATA ; 4
STA $2000,Y ; 5
LDY WDATA ; 4
STA $2000,Y ; 5
; vector to next opcode
LDA WDATA ; 4
STA @D+2 ; 4
LDA WDATA ; 4
STA @D+1 ; 4
@D:
JMP op_nop ; 3
; 4 + (4+4+5+4)+3+3+46
op_tick_18: ; XXX really tick_17
BIT tick ; 4
LDA WDATA ; 4
LDY WDATA ; 4
STA $2000,Y ; 5
BIT tick ; 4 XXX off by one - only 17 cycles
STA zpdummy ; 3
JMP _op_tick_tail_46 ; 3 + 46
;4+(4+4+5+3+4)+3+46=73
op_tick_20:
@ -550,7 +563,7 @@ op_tick_22: ; XXX really tick_21
LDY WDATA ; 4
STA $2000,Y ; 5
LDY WDATA ; 4
BIT tick ; 4 XXX off by one
BIT tick ; 4
STA zpdummy ; 3
JMP _op_tick_tail_42 ; 3 + 42
@ -646,8 +659,8 @@ op_tick_34: ; repeats from op_tick_16
@D:
JMP op_nop ; 3
;4+(4+4+5+4+5+4+5+4)+3+3+28
op_tick_36: ; repeats from op_tick_18 ; XXX really tick_35
;4+(4+4+5+4+5+4+4+2+4)+5+5+2+2+4+4+4+4+3
op_tick_36: ; repeats from op_tick_18
BIT tick ; 4
LDA WDATA ; 4
LDY WDATA ; 4
@ -655,11 +668,22 @@ op_tick_36: ; repeats from op_tick_18 ; XXX really tick_35
LDY WDATA ; 4
STA $2000,Y ; 5
LDY WDATA ; 4
STA $2000,Y ; 5
BIT tick ; 4 XXX off by one
LDX WDATA ; 4
NOP ; 2
BIT tick ; 4
STA zpdummy
JMP _op_tick_tail_28
STA $2000,Y ; 5
STA $2000,X ; 5
LDX #$00 ; 2
NOP ; 2
; used >3 pad cycles within tick loop and restoring invariant; can't branch to tail
LDA WDATA ; 4
STA @D+2 ; 4
LDA WDATA ; 4
STA @D+1 ; 4
@D:
JMP op_nop ; 3
; 4 + (4+4+5+4+5+4+5+3+4)+3+28
op_tick_38: ; repeats from op_tick_20
@ -687,7 +711,7 @@ op_tick_40: ; repeats from op_tick_22 ; XXX really tick_41
LDY WDATA ; 4
STA $2000,Y ; 5
LDY WDATA ; 4
BIT tick ; 4 XXX off by one
BIT tick ; 4
STA zpdummy
JMP _op_tick_tail_24