Mapping From Instruction Code To Microoperation Address

Mapping of Instruction

Branch Logic

Conditional Branching

• Status bits

– provide parameter information such as the carry-out from the adder, sign of a number, mode bits of an instruction, etc.

– control the conditional branch decisions made by the branch logic together with the field in the microinstruction that specifies a branch address.

• Branch Logic - may be implemented in one of several ways:

– The simplest way is to test the specified condition and branch if the condition is true; else increment the address register.

– This is implemented using a multiplexer:

• If the status bit is one of eight status bits, it is indicated by a 3-bit select number.

• If the select status bit is 1, the output is 0; else it is 0.

• A 1 generates the control signal for the branch; a 0 generates the signal to increment the CAR.

• Unconditional branching occurs by fixing the status bit as always being 1.

• Branching to the first word of a microprogram is a special type of branch. The branch is indicated by the opcode of the instruction.

• The mapping scheme shown in the figure allows for four microinstruction as well as overflow space from 1000000 to 1111111.

Читайте також:

<== попередня сторінка	\|	наступна сторінка ==>
Selection Of Address For Control Memory	\|	Computer Instructions

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