20 Oct
2012
20 Oct
'12
4:13 a.m.
On 2012-Oct-19 15:44:27 -0500, Clem Cole <clemc(a)ccc.com> wrote:
the 68000 and 68010 were 16 bit internals.
It depends whether you are talking implementation or architecture.
The M68000 architecture was basically 32 bits (Motorola initially
skimped on the multiply & divide instructions and referred to it as a
"16-/32-bit architecture"), though the initial implementation used a
16-bit ALU.
To go back further, the IBM System/360 was a 32-bit architecture but
the low-end implementation (360/20) only had 8-bit wide memory and an
8-bit wide ALU and there was also a 16-bit wide implementation.
the external logic (ie pins) supported 24 bits of
address. moto
fortunately passed all 32 bits along on the first chip and onto
storage (thank you Les & Nick) so when later versions had a full 32
bit shifter everything just worked.
They clearly defined that the programmer's view was a 32-bit address
but some implementations didn't map all the address bits onto pins.
Note that this approach of only physically implementing a subset of
the address bus has continued into the 64-bit chips - most chips only
have 36-40 physical address bits and 40-48 logical address bits.
(Though one big difference is that the unimplemented address bits are
validated instead of ignored).
PPS. we relived this whole argument with 64 bits and it
was
interesting that we generally came to think LP64 made more sense for
chips like Alpha
I think a lot of this was also driven by the large amount of software
that was ILP32. Converting int from 32- to 64-bits would add a lot
of pain for very little benefit. Just making code work with LP64 was
painful enough.
On 2012-Oct-19 16:54:28 -0400, John Cowan <cowan(a)mercury.ccil.org> wrote:
Sounds like the 8088, which used an 8-bit bus but
16-bit registers
and operations.
The 8088 and 68008 were basically 8086/68000 chips with reworked bus
interface logic so that the external data bus was only 8-bit (and the
68008 also cut the address bus from 24- to 20-bits). Other than being
slower, they appeared the same as their 16-bit cousins. They were
aimed at applications where price was more important than performance:
Using the 8088 meant that IBM only needed 8 64Kx1 DRAM chips and the
68008 used a much smaller and cheaper 40-pin DIP instead of the 64-pin
DIP needed for the 68000.
--
Peter Jeremy