Computer Architecture: Control Unit, Instruction Formats, and RISC vs. CISC, Study notes of Computer Applications

Download Computer Architecture: Control Unit, Instruction Formats, and RISC vs. CISC and more Study notes Computer Applications in PDF only on Docsity!

UNIT 2

MICROPROGRAMMED CONTROL

The function of control unit is to initiate sequence of microoperation Two methods for implementing control unit are:

• (^) Hardwired Control Unit: When the control signals are generated by hardware using conventional logic design techniques, the control unit is said to be hardwired.
• (^) Micro programmed control unit: A control unit whose binary control variables are stored in memory is called a micro programmed control unit.

Dynamic microprogramming:

• (^) A more advanced development known as dynamic microprogramming permits a microprogram to be loaded initially from an auxiliary memory such as a magnetic disk.
• (^) Control units that use dynamic microprogramming employ a writable control memory. This type of memory can be used for writing.

• (^) Control Memory: Control Memory is the storage in the microprogrammed control unit to store the microprogram.
• (^) Writeable Control Memory: Control Storage whose contents can be modified, allow the change in microprogram and Instruction set can be changed or modified is referred as Writeable Control Memory.

• (^) Microoperations: In computer central processing units, micro-operations (are detailed low-level instructions used in some designs to implement complex machine instructions (sometimes termed macro- instructions in this context).
• (^) Micro instruction: Each word in a control memory contains a micro instruction

Micro program:

• (^) A sequence of microinstructions constitutes a microprogram.
• (^) Since alterations of the microprogram are not needed once the control unit is in operation, the control memory can be a read-only memory (ROM).
• ROM words are made permanent during the hardware production of the unit.
• (^) The use of a micro program involves placing all control variables in words of ROM for use by the control unit through successive read operations.
• (^) The content of the word in ROM at a given address specifies a microinstruction.
• (^) Microcode: Microinstructions can be saved by employing subroutines that use common sections of microcode. For example, the sequence of micro operations needed to generate the effective address of the operand for an instruction is common to all memory reference instructions. This sequence could be a subroutine that is called from within many other routines to execute the effective address computation.

Address Sequencing

• (^) Microinstructions are stored in control memory in groups, with each group specifying a routine.
• (^) To appreciate the address sequencing in a micro-program control unit, let us specify the steps that the control must undergo during the execution of a single computer instruction.

Four ways of address sequencing

• (^) Incrementing of the control address register.
• (^) Unconditional branch or conditional branch, depending on status bit conditions.
• (^) A mapping process from the bits of the instruction to an address for control memory.
• (^) A facility for subroutine call and return.

• (^) An initial address is loaded into the control address register when power is turned on in the computer.
• (^) This address is usually the address of the first microinstruction that activates the instruction fetch routine.
• (^) The fetch routine may be sequenced by incrementing the control address register through the rest of its microinstructions.
• (^) At the end of the fetch routine, the instruction is in the instruction register of the computer

• (^) The next step is to generate the microoperations that execute the instruction fetched from memory.
• (^) The microoperation steps to be generated in processor registers depend on the operation code part of the instruction.
• (^) Each instruction has its own micro-program routine stored in a given location of control memory.
• (^) The transformation from the instruction code bits to an address in control memory where the routine is located is referred to as a mapping process.
• (^) A mapping procedure is a rule that transforms the instruction code into a control memory address.

• (^) Once the required routine is reached, the microinstructions that execute the instruction may be sequenced by incrementing the control address register.
• (^) Micro-programs that employ subroutines will require an external register for storing the return address.
• (^) Return addresses cannot be stored in ROM because the unit has no writing capability.
• (^) When the execution of the instruction is completed, control must return to the fetch routine.
• (^) This is accomplished by executing an unconditional branch microinstruction to the first address of the fetch routine

1. Two memory units:
   • (^) Main memory -> for storing instructions and data, and
   • (^) Control memory -> for storing the microprogram.
2. Six Registers:
   • (^) Processor unit register: AC,PC,AR,DR
   • (^) Control unit register: CAR, SBR
3. Multiplexers:
   • (^) The transfer of information among the registers in the processor is done through multiplexers rather than a common bus.
4. ALU:
   • (^) performs microoperations with data from AC and DR and places the result in AC.

• (^) DR can receive information from AC, PC, or memory.
• (^) AR can receive information from PC or DR.
• (^) PC can receive information only from AR.
• (^) Input data written to memory come from DR
• (^) Data read from memory can go only toDR.