Question

Explain mapping of instruction code to microinstruction address

Answer 1

Step 1 of 4

Opcode to micro-instruction address mapping can be done using ROM.

• The n-bits of opcode value can be used as the address input of  ROM.

• The contents of the selected word in the ROM will be the desired M-bit control address register (CAR) address for the beginning of the routine implementing that instruction.

• When an instruction is fetched from memory and loaded to IR (Instruction Register), the address lines of main memory where micro-program is stored(called micro-store) are connected to the output of the mapping ROM for each and every opcode that is loaded to IR and gives the start address for the micro-program.

• This address is sent to the micro-store and simultaneously incremented by 1 and loaded into to program counter.

• When the first word has been read, the address switch is moved to the program-counter and consecutive words of the microprogram are read at each cycle.

• At the end of the execution of each instruction, the address switch is moved to the third position and the address in the micro store of the start of the “fetch” microprogram is obtained from the micro store itself.


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Answer 2

Explanation:

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