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Each of the 68K series CPUs had dedicated address-generation hardware which was wired more directly to the A registers and had only limited access to the D registers. Conversely, the main ALU was more directly wired to the D registers than the A registers. It thus became a performance enhancement, allowing the main ALU and the addressing logic to operate ...


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The primary advantage of the 68000 segregating address and data registers is the ability to have many instructions use three-bit register-select fields, thus saving opcode space compared with using four-bit fields. IMHO, the ARM Thumb instruction set and derivatives could have benefited from employing such a concept, since otherwise the upper registers end ...


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