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Chapter 17 Micro-Programmed Control

2016-12-30 22:14 316 查看
Instuction & Microinstruction
Microinstructions: a set of micro-operations
Machine instruction: one microprogram = a set of microinstructions

Control logic is specified by a microprogram.
Microprogram: also called firmware

A sequence of instructions in a microprogramming language

A language specifying micro-operations
A clause is a microinstruction

Very simple instructions that specify micro-operations

Micro-instruction types
Vertical micro-instruction: seldom used

Each micro-instruction specifies a single(or a few) micro-operation to be performed
Width is narrow
n control signals encoded into log2 n bits
Limited ability to express parallelism

Horizontal micro-instruction

Each micro-instruction specifies many different micro-operaions to be performed in parallel
Wide memory word
High degree of parallel operations possible
Little encoding of control information

Control unit function: Should happen in one clock pulse
Sequence logic unit issues read command
Word specified in control address register is read into control buffer register
Control buffer register contents generates control signals and next address information
Sequence logic loads new address into control buffer register based on next address information from control buffer register and ALU flags

Pros & Cons
Pros:

Simplifies design of control unit

Cheaper
Less error-prone

Cons:

Slower than hardwired control

Microinstruction designing
Microinstruction sequencing

Size of microinstructions
Address generation time
How to obtain the next microinstruction

Determined by instruction register
Next sequential address
Branches

Microinstruction execution

Fetch: determined by generation of microinstruction address
Execute ( == Read): to generate control signals
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