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Processor Instruction Format

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i had to build an 8-bit processor base on RISC. As I know, one of characteristic of RISC is to have a same length of instruction. so, i used an 8-bit SRAM as a memory, and all program and data will store in this same memory. The processor should have 8 line for address and 8 line more for data.

however, i already got 24 instruction which will used 5-bit. so, how to design the instruction since the instruction is store in memory, mean it must be 8-bit width, and the address for sure need to used 8-bit. the only solution i see is to used 16-bit instruction, but it will used two memory location (and PC should increase by 2 after each instruction executed). But, there are half of the instruction is register to register operation, and this instruction can be fix into 8-bit width, is it waste to used 16-bit to hold instruction which is able to be filled in 8-bit memory. in other time, 8-bit is not enough for other half instruction.


There are machines with 8 bit instructions, but not many that I can think of. In any case, small processors like the AVR and PIC have 12 or 14 bit instructions. They use a harvard architecture which means that there are distinct data and instruction memories. The instruction memories are perhaps 1k deep by <instruction width> wide. The data memory is 8 or 16 bits or whatever. In your case, if you have a single memory and separate register file, you will need to do some clever instruction decoding.
Alternatively, you could force your design to use for instance an X and Y register for inputs and Z for output, that would eliminate some of the decoding.
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