CPU of the Day: Motorola MC68040VL

http://www.cpushack.com/2019/11/01/cpu-of-the-day-motorola-mc68040vl/

by protomyth • 7 years ago

87 23 7 years ago

Someone• 7 years ago

”It has dual MMUs”

What are dual MMUs good for? One for kernel code and one for user-mode code? (Does that mean less cache flushing during context switches?)

jacobush• 7 years ago

Guessing it's neat for I/O?

wmf• 7 years ago

I wonder if this is what we would call split TLB today: one for instructions and one for data.

jaak• 7 years ago

This is correct. It had one MMU for instructions and one for data.

tralarpa2• 7 years ago

The two MMUs use the same page table. There is only one page table root pointer (actually two: one for user mode and one for kernel mode). But there are two translation caches for code and data (usually called TLB; called ATC in the 68040).

However, there are separate configuration registers for code and data address space that can be used to define which parts of the virtual address space should be translated and which parts are directly mapped to physical addresses without translation. Therefore, strictly spoken, the instruction address space is not necessarily identical to the data address space, despite having only one page table.

tralarpa2• 7 years ago

By the way, the possible difference between instruction and data address space can have interesting effects.

Amiga programmers, do you remember the (bad) habit of putting data fields directly into the code? Like this fragment:

    ...
    jsr OpenLibrary(a6)
    move.l d0, gfxbase
    rts
    gfxbase dc.l 0

Imagine now what could happen in the above move.l instruction if the address "gfxbase" is translated differently from the addresses of the instructions :)

Koshkin• 7 years ago

[microcoding] is common in processors today and allows greater flexibility

Microprogramming was already widespread since the 1960s. The early microprocessors, such as MOS Technology’s 6502, too, used microcode (in its “hardwired” form).

jonsen• 7 years ago

In a way everything is microcoded. The 6502 was hardly microcoded in the general meaning of microcode:

“An alternate approach, used in many microprocessors, is to use PLAs or ROMs (instead of combinational logic) mainly for instruction decoding, and let a simple state machine (without much, or any, microcode) do most of the sequencing. The MOS Technology 6502 is an example of a microprocessor using a PLA for instruction decode and sequencing.”:

https://en.m.wikipedia.org/wiki/Microcode

monocasa2• 7 years ago

Having written microcode, and with the gate level information we have on the 6502 now, I can totally tell you that it's microcode. Some people get confused for some reason if they've only been exposed to vertical microcode.

perl4ever• 7 years ago

In college in the late 90s, I remember an assignment to optimize the microcode for an instruction in a hypothetical computer. I had a lot of fun doing that and came up with a much shorter solution than my professor thought possible.

I never ended up doing any low-level programming professionally though. The person I worked with that was the best at SQL, I heard hated his college course with FPGAs.

tyingq• 7 years ago

The time period is very interesting to me. The 680x0 story is pretty well documented, but their 88k chips + MVME [1] were also popular in specialized devices like firewalls. There was a time when Motorola was kicking Intel's butt in specialized areas.

[1] https://en.wikipedia.org/wiki/Motorola_88000

pavlov• 7 years ago

Motorola was even kicking ass in PCs, although you had to be selective with benchmarks to show an advantage over Intel Pentiums.

When Steve Jobs came back to Apple, he sanctioned a marketing campaign emphasizing the performance of the Motorola PowerPC G3, with Intel chips pictured on the back of snails.

amatecha• 7 years ago

One of the ads: https://www.youtube.com/watch?v=jqMruSb_SuE :)

rasz• 7 years ago

Lets not forget "the world's fastest personal computer" https://www.businessinsider.com/the-20-most-sexy-violent-and...

'The ASA has banned two separate Apple commercials from airing in the UK. In 2004 the Power Mac G5 made unsubstantiated claims of being "the world's fastest personal computer" and in 2008, an iPhone commercial was banned for saying that it could access "all of the internet" even though it couldn't support Flash.'

classichasclass• 7 years ago

Here's an MCF5102 based system, the Alpha Micro Eagle 450. Alpha Micro used the MCF5102 in their ColdFire-based systems for its '040 backwards compatibility.

http://ampm.floodgap.com/www/e450.htm

watersb• 7 years ago

Big fan of your work.

Once I get my desk rebuilt I will fire up my PowerPC collection again.

cmrdporcupine• 7 years ago

There's an over $5000 bounty for someone to rewrite the GCC backend for the 68k/ColdFire so it can be kept and maintained going forward for newer versions.

https://www.bountysource.com/issues/80706251-m68k-convert-th...

CPU of the Day: Motorola MC68040VL

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