Should we have a jump register? #19
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freesurfer-rge
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Oct 14, 2022
After some thought, it feels better to have a dedicated Jump register built into the program counter. This helps deal with the design having 8-bit registers but using 16-bit addressing, which had lead to needing the instruction to copy the PC to be able to write to *two* registers. It was the only instruction which needed to do so, and complicated the register file. Having a jump register avoids this, since it is safe to copy the low and high bytes with separate instructions. I believe that this compensates for the extra instructions now in the Program Counter. Closes #19
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Should the
ProgramCounteracquire a 16-bit jump register, along with instructions:JSRwhich would copy the current PC to the jump register, and copy A and B bus to the PC (and inhibit increment)RETwhich would copy the jump register to the PC (and not inhibit increment)LOADJUMP0/LOADJUMP1which would put the low/high byte of the jump register onto C bus (for saving in Register C)STOREJUMPwhich would copy A and B bus to the jump registerThese would eliminate the
LOADPCcommand, which is the only one to write to Register B.... and complicates theRegisterFileimplementation. Since the jump register wouldn't be continually changing, it is safe to have separate load instructions for the low and high bytes.The text was updated successfully, but these errors were encountered: