Chapter 17 Micro-Programmed Control
2016-12-30 22:14
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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
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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