Design Debug Upgrade
Microprogrammed Control
Hardwired control units have the advantage of great compactness and low propagation delay on account of the shorter loop path.
However as the number of states and control signals increases they become increasingly costly to:
A more flexible approach is used.
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The Flexible Approach
We store the control words, together with next-state information , in a control memory which is usually:
ROM EPROM
Then use the control inputs and status signals to select the appropriate address of the next state.
See figure on the next slide.
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Microprogrammed Control Unit Organization
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IDLE • G=0 IDEL• G=1
none CW1 C,A0 CW2 none CW3 AA+B, CCout CW4
MUL0 • MUL0 • more
Q0=0 Q0=1
Control Input
One difference which emerges is that since control words are now stored, they cannot be made to depend dynamically on the on the value of the control input
STATE Control Input
RT
Control Word
more
more
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Binary Multiplier ASM
Hence we must introduce additional states to supply these alternative control words.
See next slide
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Microprogrammed Control Unit Binary Multiplier
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Control Address Register (CAR) This gives five states so that the control
memory must store five control words We will need a 3-bit address register
Control Address Register (CAR)
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SEL bits
The sequence must be able to respond to
two status bits:
Z
Q0
One control input:
G
Hence two SEL bits must be included in the control word.
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Next-Address Fields
Finally we add two next-address fields: NXTADD0
NXTADD1
This design makes no assumption about the sequence control word accesses.
The functional design of the sequence is as follows on the next slide:
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SEL Field Definition
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Control Signals for Multiplier control
The control word must supply four control signals:
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Control Word
This results in a 12-bit control word:
11 98 65 43210 NXTADD1 NXTADD0 SEL SD CC LD IT
Control Datapath
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Control Unit
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Register Transfer Description
Next we design the microprogram in symbolic RT form:
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Symbolic Microprogram & Binary Microprogram
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