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Selection Of Address For Control Memory

Addressing Sequencing

Mapping

Address Sequencing

Sequencer

• The sequencer generates a new address by:

– incrementing the CAR

– loading the CAR with an address from control memory.

‑ transferring an external address

or

‑ loading an initial address to start the control operations.

• Microinstructions are usually stored in groups where each group specifies a routine, where each routine specifies how to carry out an instruction.

• Each routine must be able to branch to the next routine in the sequence.

• An initial address is loaded into the CAR when power is turned on; this is usually the address of the first microinstruction in the instruction fetch routine.

• Next, the control unit must determine the effective address of the instruction.

• The next step is to generate the microoperations that executed the instruction.

– This involves taking the instruction’s opcode and transforming it into an address for the the instruction’s microprogram in control memory. This process is called mapping.

– While microinstruction sequences are usually determined by incrementing the CAR, this is not always the case. If the processor’s control unit can support subroutines in a microprogram, it will need an external register for storing return addresses.

• When instruction execution is finished, control must be return to the fetch routine. This is done using an unconditional branch.

• Addressing sequencing capabilities of control memory include:

– Incrementing the CAR

– Unconditional and conditional branching (depending on status bit).

– Mapping instruction bits into control memory addresses

– Handling subroutine calls and returns.

Читайте також:

1. Addressing Modes
2. Control Unit Design
3. Control work
4. Mapping From Instruction Code To Microoperation Address
5. Microprogram Sequencer For A Control Memory
6. Microprogrammed Control Organization
7. Program Control
8. Оперативна пам'ять RAM (Random Access Memory).

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