## Re: A86: simple math optimization

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# Re: A86: simple math optimization

```
At 16:52 1998-05-11 -0500, you wrote:
>> What's the fastest way to do
>>
>> hl = (16 * d) + e

>ld   (D_temp), d  ; 13
>ld   hl, D_temp   ; 10
>xor  a            ;  4
>rld               ; 18
>ld   d, a         ;  4
>ld   a, (D_temp)  ; 13
>add  a, e         ;  4
>ld   l, a         ;  4
>ld   a, d         ;  4
>adc  a, 0         ;  7
>ld   h, a         ;  4

Slow - 85 T cycles. The idea could be optimized though

>ld  h, 0
>ld  l, d
>add hl, hl
>add hl, hl
>add hl, hl
>add hl, hl        ; now hl = 16 * d
>ld  d, 0
>add hl, de        ; and then we add the e

That would be  7+4+11*4+7+11 = 73 T cycles, but could be made 3 T cycles
faster if "ld d,0" was replaced by "ld d,h" after the "ld l,d" instruction.

This is faster (fastest?)

ld a,d   ; 4
rlca     ; 4
rlca     ; 4
rlca     ; 4
rlca     ; 4
ld l,a   ; 4
and 15   ; 7
ld h,a   ; 4
ld a,l   ; 4
xor h    ; 4
or e     ; 4
ld l,a   ; 4

51 T cycles

This assumes E<16, but I guess that is the case (else 16*D+E is a strange
operation to make). Could be changed to support E>=16 as well, without
losing that many T cycles.

--
Real name: Jimmy Mårdell                "can't go mucking with a 'void *'"
IRC......: Yarin                                // Apple's MPW C compiler
Email....: mailto:yarin@acc.umu.se
Homepage.: http://www.algonet.se/~mja/

```

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