[sdiy] uC with mul

[Jim Patchell]

     Here is a thought for doing an 8x8 multiply...use a look up table.  
If you use a 128Kx8 ROM, you can put a look up table in it to make an 
8x8=16bit multiplier.  All you need are two 8 bit registers, and then 
use one of the address lines to access the MSB and LSB parts of the results.

   Anyway, just a thought.

   -Jim

Scott Gravenhorst wrote:

> I did quite a bit of assembly programming with a MC6800
> MPU (not 68K !) which has no mult instruction at all.
> There are cases where you need to multiply a variable
> number by a constant.  In these cases, a short sequence
> of shifts and adds does the trick and can be faster than
> using a generic multiply subroutine.  So if you are 
> stuck with a no-mult mcu, or if you have 8x8 and need
> 8x16 or 16x16, this technique can be useful as well.
> Much depends on how fast the builtin 8x8 is.
> 
>>> -----Original Message-----
>>> From: ext John Loffink [mailto:jloffink at austin.rr.com]
>>> Sent: 30. December 2001 17:08
>>> To: Byron G. Jacquot; synth-diy at dropmix.xs4all.nl
>>> Subject: RE: [sdiy] uC with mul
>>> 
>>> 
>>> The multiply cycle on the 68HC16 is 8 cycles.  Other micros 
>>> do have the
>>> multiply instruction, usually multi cycle instructions.  
>>> These might be
>>> good enough depending on your application, particularly for low
>>> frequency stuff like note generation, envelopes, LFOs, 
>>> control sources,
>>> etc., but for anything related to analog synthesis it's 
>>> generally better
>>> to have 12-16 bits of resolution to avoid stair-stepping.
>>> 
>>> John Loffink
>>> jloffink at austin.rr.com 
>>> 
>>> -----Original Message-----
>>> From: owner-synth-diy at dropmix.xs4all.nl
>>> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Byron 
>>> G. Jacquot
>>> Sent: Saturday, December 29, 2001 10:29 PM
>>> To: synth-diy at dropmix.xs4all.nl
>>> Subject: RE: [sdiy] uC with mul
>>> 
>>>> Motorola M68HC16 microcontroller series has a 16 bit
>>>> multiplier/accumulator.
>>> 
>>> The "little brother" 68HC11 could also do 8*8 multiply, and 
>>> (IIRC) 16/8
>>> division.  They were both kinda slow.  I think a multiply took 12
>>> cycles,
>>> and the divide something amazing like 42.
>>> 
>>> Byron Jacquot
>>> 
> 
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