15 Aug
2024
15 Aug
'24
8:52 p.m.
On Aug 15, 2024, at 4:41 PM, Sean Conner via cctalk
<cctalk(a)classiccmp.org> wrote:
It was thus said that the Great ben via cctalk once stated:
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
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