Re: Macintosh LC...need info on this =) TIA

[Roy and/or Janet Miller <millers@inetnebr.com>]

Matt Jenkins wrote:

> Roy and/or Janet Miller <millers@inetnebr.com> wrote:

>
> > The 68000 is generally rated as a 16/32 bit chip (kinda like the 8088 was an
> > 8/16 bit chip)
>
> I think you mean 32/24 bit chip.

No Matt, I meant 16/32. The 68000 had a 16 bit data bus, as the 8088 had an 8 bit data bus.
The 68000 had 32 bit registers (with, as you mention, a 24 bit address space) while the
8088 had 16 bit registers with a 20 bit address space. In a similar fashion the 386SX was a
16/32 chip: 16 bit data bus, 32 bit registers, 24 bit address space.

> The 68000 has real 32 bit instructions, real 32 bit
> registers, etc. It is indeed a 32 bit processor. The only 'problem' is that the address
> bus of the thing can only use 24 bits to address memory, and hence you're limited to
> 16mb of ram on a 68k.
>
> This is where the Mac concept of '32-bit clean' apps comes from. Many old applications
> (including MacOS 6.x) used to hide data in the high order byte of the 68k's address
> registers for speed, since only 24 bits were actually used for addressing. This is why
> 68030 Macs (eg. Mac IIci, IIsi, LCIII) have the 32bit addressing switch in the control
> panel, so you can disable 32bit addressing and run older applications or older System
> software. Starting with the 68040 machines (Quadra, Centris, etc) the CPU's could no
> longer do this, and hence you couldn't run any OS older than 7.0, and dozens of old
> classic Mac apps also.
>
> Hope this clears things up - too much Mac lore for a II news group :)

No argument with you on that one Matt.

Roy

>
>
> --
>               Matt(hew) Jenkins. Computer Science (RMIT Bundoora)
>                            email: mdj@cs.rmit.edu.au
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