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| Author | 68000 16-bit or 32-bit?? (Read 29968 times) | ||||||||||||||
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snes1423
256 MB  Posts: 458 A Man born of Mechina 
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on: April 22, 2025, 00:45
so ive heard that the 68000 chip can be either 16-bit or 32-bit is this true and how is that? |
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MTT
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256 MB Posts: 394 SSW7 Oldtimer
Reply #1 on: April 22, 2025, 07:43
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The Wikipedia page on the Motorola 68000 CPU gives a good background description.
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lauland
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512 MB Posts: 674 Symtes 7 Mewconer!
Reply #2 on: April 22, 2025, 16:05
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Short summary: Looks like 16 bits, tastes like 32 bits. Runs 32 bits code inside, but has a 16 bit data bus outside. The address bus is also limited to 24 bits, for a max of 16m address space. (Which is why machines using a 68000 tend to max out at 8m of ram, the other 8m is used for rom, I/O and expansion space). This was done for many reasons, the size and manufacturing of the chip using the technology of the day. Cost, as always. The ease of using existing 16 bit support chips and designs, etc. But it meant you could write 32 bit code and run it. It would just take two memory reads when you accessed 32 bit quantities. This is one of the reasons why in early Mac high level language compilers, the basic "integer" type was 16 bits by default, though most could optionally use 32 bits. The 68020 was fully 32 bits, but data and address bus. Trivia: Even the first PowerPC chip had a 64 bit data bus, but only ran 32 bit code. This was done for the opposite reason the 68000 used a 16 bit bus: They read 64 bits at a time from their caches.
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lauland
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512 MB Posts: 674 Symtes 7 Mewconer!
Reply #3 on: April 28, 2025, 16:20
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Nice video "68000 - The CPU ahead of its time": https://www.youtube.com/watch?v=njGWWg69B4A (modern link)
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