CSU22022, 13th Lecture, Dr. M. Manzke, Page: 1
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:
Design
Debug
Upgrade
A more flexible approach is used.
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 2
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.
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 3
Microprogrammed
Control Unit Organization
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 4
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
IDLE • G=0 none CW1
IDEL • G=1 C, A 0 CW2
MUL0 • Q0=0 none CW3
MUL0 • Q0=1 AA+B, CCout CW4
more more more
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 5
Binary Multiplier ASM
Hence we must
introduce
additional states
to supply these
alternative
control words.
See next slide
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 6
Microprogrammed Control Unit
Binary Multiplier
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 7
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)
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 8
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.
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 9
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:
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 10
SEL Field Definition
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Control Signals for Multiplier
control
The control word must supply four
control signals:
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 12
Control Word
This results in a 12-bit control word:
NXTADD1 NXTADD0 SEL SD CC LD IT
Control Datapath
01234568911
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 13
Control Unit
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 14
Register Transfer Description
Next we design the microprogram in
symbolic RT form:
CSU22022, 13th Lecture, Dr. M. Manzke, Page: 15
Symbolic Microprogram
& Binary Microprogram