Going through the system bus IS an implementation detail.
You could build a 6502-compatible CPU with a (extra [1]) 256 byte on-chip register file, and treat, for example, `0x1265` as simply a 16 bit instruction `ADC A,R18`, or `0x0791` as an x86-ish `MOV [R7+Y],A`.
All binary programs would run just as they do on the 1975 6502, just a lot faster.
[1] in the original 6502, the registers aren't in a register file in the modern sense, they're implemented with flip flops and all are accessible simultaneously (with wired-OR on to a bus in some cases if the decode ROM selected several at the same time)
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Going through the system bus IS an implementation detail.
You could build a 6502-compatible CPU with a (extra [1]) 256 byte on-chip register file, and treat, for example, `0x1265` as simply a 16 bit instruction `ADC A,R18`, or `0x0791` as an x86-ish `MOV [R7+Y],A`.
All binary programs would run just as they do on the 1975 6502, just a lot faster.
[1] in the original 6502, the registers aren't in a register file in the modern sense, they're implemented with flip flops and all are accessible simultaneously (with wired-OR on to a bus in some cases if the decode ROM selected several at the same time)