>Usually the size of general purpose registers is what defines the bitness of a CPU

The Motorola 68000 has 32-bit registers but it's usually considered a 16-bit CPU because it has 16-bit ALU and 16-bit data bus (both internal and external).

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p_l 5 days ago | parent [-]

Motorola 68k is a curious case because it originally was supposed to be a 16bit cpu, not 32bit, and the 24bit addressing that ignored upper 8 bits didn't help the perception.

Ultimately, 68k being "16bit" is a marketing thing from home computers that upgraded from 8bit 6502 and the like to m68k but didn't use it fully.

	▲	dspillett 5 days ago \| parent \| next [-]
		That is an odd case. I'd still call it a 32-bit CPU as it had 32-bit registers and instructions (and not just a few special case 32-bit instructions IIRC). Like the 386SX it had a 16-bit external data bus, but some of its internal data routes were 16-bit also (where the 386SX had the full 32-bit core of a 386, later renamed 386DX, with the changes needed to change the external data bus) as were some of its ALUs hence the confusion abaout its bit-ness.
	▲	ddingus 5 days ago \| parent \| prev [-]
		Well, the 6809 was basically the same in these respects. Internal registers are 16 bit, with the accumulator (A) being provisioned as two 8 bit registers (A, B) as needed. Index X, Y, Stack, User Stack, PC, are all 16 bit registers. The Hitachi 6309, adds to that with up to 32 bit register sizes in specific cases. In any case, the ALU and data transfers are 8 bits and I am not sure I ever saw the 6809 referenced as a 16 bit device. Maybe 16 bit curious, LMAO.