Macintosh LC Hardware Book Recommendations

[Arbee]

Floppy is pretty standard if you have SWIM1 implemented. The ROM will detect it and work if you substitute an IWM and an 800k drive, as well, and since the LC can boot System 6 it's even feasible to boot that way.

SCSI is the same as every other Mac with a 5380 since the Plus, including the same "pseudo-DMA" that can freeze the processor temporarily. So if you borrow that from a Mac Plus (which I'm pretty sure there's a Mister core for already) it oughta work.

Motorola's MMU was extremely over-designed (not as much as the i386's though!) and got progressively simpler each generation of 68K.

 

[Snial]

<snip> ROM will detect it and work if you substitute an IWM and an 800k drive, as well, and since the LC can boot System 6 it's even feasible to boot that way.

I didn't know that! LC II can also boot System 6.0.8!

<snip> "pseudo-DMA" that can freeze the processor temporarily<snip>

Not so different to the Q630 then!

Motorola's MMU was extremely over-designed (not as much as the i386's though!) and got progressively simpler each generation of 68K.

This has been noted. @Phipli was correct to point out the MMU. This was my main criteria when buying my first Mac, my LC II (Performa 400 actually). I didn't want the software I was running to be dependent on my physical RAM limit (4MB at first, 8MB at the end). I don't think I appreciated at the time how much of a performance hit Apple's VM implementation was.

I find it a bizarre decision that MMUs started out complex and became simpler over time (and x86 had Segmentation too!). If I had been designing MMUs for early 32-bit CPUs, I would have started with the simplest one I could: a user-mode only MMU with maybe 16x 32-bit TLBs and a simple linear page table on a 4kB boundary. I'd do that because I'd assume that OS software would want to start with a simple VM model so that it could be implemented quickly. Later I'd make it more comprehensive.

ASID<4>

Access<2> [Code, Read-only, Read-Write-Unmodified, Read-Write-Modified

Tag<14>

Translation<12>

ASID (Address-space identifier) bits here are just copied from the MIPs R3000. A 14-bit tag/12-bit translation provides a 64MB Virtual address space mapped to a 16MB physical address space.

Some MMU's were fairly simple. The original ARM2 MMU (MEMC) was a strange physical to virtual map: an array of 128 entries for managing 128 physical pages each containing its virtual tag. It could manage up to 4MB of physical space within a 32MB virtual space. Pages were 32kB at 4MB, down to 4kB for a 512kB Archimedes.