William Stallings Computer Organization and Architecture 6 th

William Stallings Computer Organization and Architecture 6 th Edition Chapter 17 Microprogrammed Control

Structure - The Control Unit CPU Sequencing Logic ALU Internal Bus Registers Control Unit Registers and Decoders Control Memory

Microinstruction Table 16. 1

Microinstructions q Each micro-operation is described in symbolic notation q A notation that describes a set of microoperations occurring at one time is called a microinstruction q A sequence of microinstructions is a microprogram, or firmware q A microprogram is midway between hardware and software

Using Microprogramming in Control Unit q Each control line from the control unit can be represented by a binary bit q All the control lines construct a control word q Each micro-operation can be represented by a different control word q Put a sequence of control words into a memory, give each word an unique address, and a way to determine the address of next control word to be executed

Microinstruction Formats

Horizontal vs. Vertical

Execute a Microinstruction q To execute a microinstruction, turn on all the control lines indicated by a 1 bit • Cause one or more micro-operations to be performed q If the condition bit is false, execute the next microinstruction in sequence q If the condition bit is true, the next microinstruction to be executed is indicated in the address field

Organization of Control Memory • • • Jump to indirect or execute • • • Jump to fetch Jump to opcode routine • • • Jump to fetch or interrupt • • • Fetch cycle routine Indirect cycle routine Interrupt cycle routine Execute cycle routine ADD routine • • •

Control Unit Microarchitecture

Key Elements of Control Unit q Control memory • Store microinstructions q Registers • Control address register • Control buffer register q Sequencing logic • Loads control address register • Issues READ command to control memory

Fig 17. 4 Functioning of Microprogrammed Control Unit

Wilkes’s Microprogrammed Control Unit

Adv. and Disadv. of Mircoprogramming q Advantage • Simplifies the design of the control unit • Cheaper • More reliable q Disadvantage • Slower q Micropragrammed control unit is used in CISC processors q Hardwired implementation is used in RISC processors