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Useful Routines

» Forum > z80 Assembly
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Xeda112358

Power User

Joined: 09 Nov 2010
Posts: 345

Posted: 03 Jul 2012 11:10:04 am Post subject:

Here is another routine that I made yesterday for a pseudo-random number generator: Code: PseudoRandWord: ;Outputs: ; BC was the previous pseudorandom number ; HL is the pseudorandom number ;f(n+1)=(241f(n)+257) mod 65536 ;65536 ;181 cycles, add 17 if called ld hl,(randSeed) ld c,l ld b,h add hl,hl add hl,bc add hl,hl add hl,bc add hl,hl add hl,bc add hl,hl add hl,hl add hl,hl add hl,hl add hl,bc inc h inc hl ld (randSeed),hl ret There are a few other nice features, too. For example, every 16-bit value is hit if you run this 65536 times. Or, if you only read 1 byte (for example, H from the output), it will hit every 8-bit number once if you run this 256 times. Plus, it can be seeded

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Xeda112358

Power User

Joined: 09 Nov 2010
Posts: 345

Posted: 07 Mar 2013 05:28:50 pm Post subject:

I was optimising some algorithms and code in Grammer and I came across a pretty big speed optimisation for computing 16-bit GCD: Code: GCDDE_HL: ;Inputs: ; HL,DE are the two values ;Outputs: ; B is 0 ; DE is 0 ; HL is the GCD ; C is not changed ; A is not changed ld b,1 or a CheckMax: ; sbc hl,de ;ED52 15n jr z,AdjustGCD ;28 12n-5 jr nc,ParityCheck ;30 12n-5 add hl,de or a ex de,hl ParityCheck: ; bit 0,e ;CB 8a jr nz,DE_Odd ;20 12a-5b bit 0,l ;CB 8b jr z,BothEven ;28 12b-5c rr d ;CB 8(n-a-b-c) rr e ;CB 8(n-a-b-c) jp CheckMax ;C3**** 10(n-a-b-c) BothEven: ; inc b ;04 4c rr d \ rr e ; 16c HL_Even: rr h \ rr l ; 16c jp CheckMax ; 10c DE_Odd: ; bit 0,l ; 8b jr z,HL_Even ; 12b-5d sbc hl,de ; 15d rr h \ rr l ; 16d jp nz,CheckMax ; 10d AdjustGCD: ; ex de,hl ; 4 dec b ; 4 ret z ; 11+4(k>0) add hl,hl ; 11k djnz $-1 ; 13k-5 ret ; -- I was using a slightly more naive approach before, computing the mod of two 16-bit values over and over (the Euclidean Algorithm). This code is pretty hefty, though, at 61 bytes. You can save three bytes by turning the JP instructions into JR, but there will be a tiny speed loss of a few cycles

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