[Info-vax] Integer Divide, Multiply, Alpha SRM(was: Re: Kednos PL/I)

[Stephen Hoffman]

On 2023-09-15 03:17:31 +0000, gah4 said:

> On Thursday, September 14, 2023 at 7:38:58 AM UTC-7, Johnny Billquist wrote:
> 
> (snip)
> 
>> It's UMULH, but I agree. The Alpha was definitely designed for getting 
>> a 128 bit result from 64 bit multiply. But it is done via two 
>> instructions. MULQ;UMULH. If you want a signed MUL, you then need a 
>> couple of more steps to adjust the result. But it's a simple 
>> thing.Okay, I found my "Alpha Architecture Handbook", copyright 1992.
> 
> I think I got it before machines were available for sale.
> 
> And what I had forgotten, and might have known 31 years ago:
> There is no hardware integer divide.
> For divide by constant, you can usually, or maybe always, use the
> high half of the double length product.
> And for variable divide, they suggest, but don't write out, a successive
> approximation algorithm.  Presumably usually as a subroutine call.
> 
> There is floating point divide, though.

Invert and multiply, quite commonly.
See A.4.2 in SRM 
http://bitsavers.org/pdf/dec/alpha/system_reference/Alpha_System_Reference_Manual_Version_5_199205.pdf 

The Alpha Architecture Reference Manual (ARM) was a de-tuned version of 
the Alpha System Reference Manual (SRM).

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