[sdiy] uC with mul

[Scott Gravenhorst]

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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