Byte sizes (was Re: 2.8M 3.5' floppy
bfranchuk at jetnet.ab.ca
Mon Mar 14 11:27:51 CST 2005
der Mouse wrote:
>Well, it sounds reasonable, but probably isn't the most useful, as it
>would mean that, for example, the PDP-8 had 12-bit bytes.
Umm words... it has six bit bytes. I think the lack of 18 bit byte
cpu's has limited
the use of 9 bit bytes. ( No comment from any 36 bitters )
>>The PDP-10 is an excellent example of when this isn't true.
>>The smallest addressable unit is a word, which is 36 bits.
>>A byte is, as noted, anything between 0 and 36 bits. Bytes are
>>stored in a word, as many as can be fitted. To access bytes on a
>>PDP-10, you have a byte pointer, which consist of a word address, and
>>a bit pointer, and byte size.
>That sounds a whole lot like a hardware-supported way of addressing an
>object of an arbitrary size in bits. And that would mean that bytes of
>any size *are* individually addressible.
>Or have I misunderstood?
I belive so, but I have not programmed a 10. I suspect this is in
regards to string operations
rather say accessing character sized data data. Two different things.
>>The fact that people today seem to believe that byte addressable is
>>the only possible thing, along with a byte being 8 bits, is plain and
>>simply because they haven't seen any other.
Well in most cases a byte is a unsigned 1/2 half word. I still like view
of the PDP-11 a byte
is signed data. I think the lack of real byte access has forced the C
standard to have unsigned
bytes because the machines ( 8080,Z80) can't handle real bytes .
>>There even are relatively modern machines which don't fit it very well.
>>Some DSPs, for example, have 32 bits as their smallest directly
It is word access I found the problem , not byte acess in the smaller
machines a problem.
As for the 386(+) I can say it still needs 8080 style code generation
for simple compilers
and compilers provide it. IE pesudo code.
LD A,(FOO); LD B,BYTE(BAR);CLR B_HIGH;ADD A,B;ST A,(FOOBAR);
Ben alias woodelf
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