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6

TL;DR Too complicated for not much benefit. You ask why not include an execute instruction. The reason is quite simple. Since around the time you observed the absence of this kind of instruction, the CPU's got more and more optimized in their memory access. Code access was separated from data access as the access pattern are quite different. To execute a ...


9

The HP-3000 first introduced in 1972 was a 16-bit stack-based architecture that included an XEQ instruction that would treat a word on the stack (between TOS and 7 words below that as selected in the instruction) as a regular instruction and execute it. This was utilized in some calling conventions where you needed to execute a different version of the EXIT ...


22

The opcodes are already sorted that way. Just a bit less obvious and schoolbook-like, but optimized to allow compact decoding. It is all about space saving. Real chip space and (potential) transistors that is. It's well known that the 6500 design was all about cost saving and the most important factor in chip production cost is its size. Smaller chips mean ...


38

The instruction decode is quite simple on the 6502. If we call the bits in the opcode byte aaabbbcc, then one of the first things that happens is that cc, the two bits you're talking about, gets converted into a 1-of-3 signal which selects the register. This signal is called G, and is computed like this: A is true if the bits are 01 X is true if the bits ...


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