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peasant: looking into random number generator performance

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Vince Weaver 2024-10-09 16:28:56 -04:00
parent 809f40c4c2
commit ea011a495a
1 changed files with 24 additions and 82 deletions
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106
games/peasant/random16.s
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@ -1,9 +1,9 @@
; 16-bit 6502 Random Number Generator (cycle-invariant version)
; 16-bit 6502 Random Number Generator
; Linear feedback shift register PRNG by White Flame
; http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
; The Apple II KEYIN routine increments SEEDL:SEEDH
; The Apple II KEYIN routine increments this field
; while waiting for keypress
;SEEDL = $4E
@ -15,105 +15,47 @@ XOR_MAGIC = $7657 ; "vW"
; random16
;=============================
; takes:
; not 0, cs = 6(r16)+12(lnz)+5(nop)+ 19(deo) = 42
; not 0, cc = 6(r16)+14(lnz)+2(nop)+ 20(neo) = 42
; $0000 = 6(r16)+ 6(loz)+11nops+ 19(deo) = 42
; $8000 = 6(r16)+ 6(loz)+ 4(ceo) + 6nops+ 20(neo) = 42
; $XX00 cc = 6(r16)+ 6(loz)+4(ceo)+2(cep) +4nops+ 20(neo) = 42
; $XX00 cs = 6(r16)+ 6(loz)+4(ceo)+4(cep) +3nops+ 19(deo) = 42*
; not 0, cc = 5+ = 27
; not 0, cs = 5+12+19 = 36
; $0000 = 5+7+19 = 31
; $8000 = 5+6+14 = 25
; $XX00 = 5+6+7+19 = 37
random16:
lda SEEDL ; 3
beq low_zero ; $0000 and $8000 are special values ; 3
;==========
; 6
lownz:
; -1
beq lowZero ; $0000 and $8000 are special values ; 2
asl SEEDL ; Do a normal shift ; 5
lda SEEDH ; 3
rol ; 2
bcs five_cycle_do_eor ; 3
;===========
; 12
bcc noEor ; 2
bcc two_cycle_no_eor ; 3
;==========
; 12+3-1 = 14
;===================================================================
eleven_cycle_do_eor:
nop ; 2
nop ; 2
nop ; 2
five_cycle_do_eor:
nop ; 2
three_cycle_do_eor:
sta SEEDH ; nop ; 3
do_eor:
doEor:
; high byte is in A
eor #>XOR_MAGIC ; 2
sta SEEDH ; 3
lda SEEDL ; 3
eor #<XOR_MAGIC ; 2
sta SEEDL ; 3
eor_rts:
rts ; 6
;===========
; 19
;=========================================================================
six_cycles_no_eor:
nop ; 2
four_cycle_no_eor:
nop ; 2
two_cycle_no_eor:
nop ; 2
no_eor:
nop ; 2
nop ; 2
nop ; 2
nop ; 2
sta SEEDH ; 3
jmp eor_rts ; 3+6
;===========
; 20
;======================================================================
;======================================================================
low_zero:
lowZero:
; 1
lda SEEDH ; 3
beq eleven_cycle_do_eor ; High byte is also zero ; 3
; so apply the EOR
;============
; 6
ceo:
beq doEor ; High byte is also zero ; 3
; so apply the EOR
; -1
; wasn't zero, check for $8000
asl ; 2
beq six_cycles_no_eor ; if $00 is left after the shift; 3
; then it was $80
;===========
; 4
; else, do the EOR based on the carry
cep:
; -1
bcc four_cycle_no_eor ; 3
;============
; 2
bcs three_cycle_do_eor ; 2+3-1 = 4
beq noEor ; if $00 is left after the shift ; 2
; then it was $80
bcs doEor ; else, do the EOR based on the carry ; 3
noEor:
; 1
sta SEEDH ; 3
lda SEEDL ; always return SEEDL
rts ; 6