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a8ab24004d
need to change a bunch to make this work Now load at $4000 as was too big to fit between $1000 and $2000 also turns out our lz4 code was writing past the end, which you notice when you have important code right after the end of HGR1
120 lines
2.5 KiB
ArmAsm
120 lines
2.5 KiB
ArmAsm
; 16-bit 6502 Random Number Generator (cycle-invariant version)
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; Linear feedback shift register PRNG by White Flame
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; http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
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; The Apple II KEYIN routine increments SEEDL:SEEDH
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; while waiting for keypress
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SEEDL = $4E
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SEEDH = $4F
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XOR_MAGIC = $7657 ; "vW"
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;=============================
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; random16
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;=============================
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; takes:
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; not 0, cs = 6(r16)+12(lnz)+5(nop)+ 19(deo) = 42
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; not 0, cc = 6(r16)+14(lnz)+2(nop)+ 20(neo) = 42
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; $0000 = 6(r16)+ 6(loz)+11nops+ 19(deo) = 42
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; $8000 = 6(r16)+ 6(loz)+ 4(ceo) + 6nops+ 20(neo) = 42
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; $XX00 cc = 6(r16)+ 6(loz)+4(ceo)+2(cep) +4nops+ 20(neo) = 42
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; $XX00 cs = 6(r16)+ 6(loz)+4(ceo)+4(cep) +3nops+ 19(deo) = 42*
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random16:
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lda SEEDL ; 3
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beq low_zero ; $0000 and $8000 are special values ; 3
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;==========
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; 6
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lownz:
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; -1
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asl SEEDL ; Do a normal shift ; 5
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lda SEEDH ; 3
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rol ; 2
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bcs five_cycle_do_eor ; 3
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;===========
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; 12
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bcc two_cycle_no_eor ; 3
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;==========
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; 12+3-1 = 14
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;===================================================================
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eleven_cycle_do_eor:
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nop ; 2
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nop ; 2
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nop ; 2
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five_cycle_do_eor:
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nop ; 2
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three_cycle_do_eor:
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sta SEEDH ; nop ; 3
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do_eor:
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; high byte is in A
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eor #>XOR_MAGIC ; 2
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sta SEEDH ; 3
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lda SEEDL ; 3
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eor #<XOR_MAGIC ; 2
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sta SEEDL ; 3
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eor_rts:
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rts ; 6
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;===========
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; 19
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;=========================================================================
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six_cycles_no_eor:
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nop ; 2
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four_cycle_no_eor:
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nop ; 2
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two_cycle_no_eor:
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nop ; 2
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no_eor:
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nop ; 2
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nop ; 2
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nop ; 2
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nop ; 2
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sta SEEDH ; 3
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jmp eor_rts ; 3+6
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;===========
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; 20
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;======================================================================
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;======================================================================
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low_zero:
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lda SEEDH ; 3
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beq eleven_cycle_do_eor ; High byte is also zero ; 3
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; so apply the EOR
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;============
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; 6
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ceo:
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; -1
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; wasn't zero, check for $8000
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asl ; 2
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beq six_cycles_no_eor ; if $00 is left after the shift; 3
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; then it was $80
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;===========
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; 4
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; else, do the EOR based on the carry
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cep:
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; -1
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bcc four_cycle_no_eor ; 3
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;============
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; 2
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bcs three_cycle_do_eor ; 2+3-1 = 4
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