> On Aug 15, 2024, at 4:41 PM, Sean Conner via cctalk <cctalk@classiccmp.org>
> wrote:
>
> It was thus said that the Great ben via cctalk once stated:
>>
>> I don't know about the VAX,but my gripe is the x86 and the 68000 don't
>> automaticaly promote smaller data types to larger ones. What little
>> programming I have done was in C never cared about that detail.
>> Now I can see way it is hard to generate good code in C when all the
>> CPU's are brain dead in that aspect.
>>
>> char *foo, long bar;
>> ... foobar = *foo + bar
>> is r1 = foo
>> r3 = * r1
>> r2 = bar
>> sex byte r3
>> sex word r3
>> r4 = r3 + r2
>> foobar = r3
>> what I want is
>> bar = * foo + bar
>> nice easy coding.
>
> What CPUs did it correctly? And how did they handle signed vs. unsigned
> promotion?
>
> unsigned char *ufoo;
> unsigned long ubar;
>
> ufoobar = *ufoo + ubar;
>
> signed char *foo;
> signed long bar;
>
> foobar = *foo + bar;
>
> -spc
Obviously, "correctly" is in the eye of the beholder. You can do size
extension, signed or unsigned, on any computer. How complicated it is depends
on the machine.
For example, on VAX there are instructions for signed as well as unsigned
promotion (CVTxy and MOVZxy respectively). On PDP11, MOVB into a register does
sign extension; unsigned promotion requires two instructions but that's no big
deal. And of course, promotion to bigger types requires multiple instructions
either way since you're now dealing with multiple registers.
Unsigned promotion on a CDC 6600 is one instruction; signed requires three.
paul
