Micro-programmed State Machines - Digital Design of Signal Processing Systems: A Practical Approach - page 433

1

+

addr bus

Microprogram Memory

N

0

cond 0
cond 1

M

data bus

2 cond

sel

branch
addr

cntr
signals

parity
bit

1

to datapath

N

1

from datapath

1

Figure 10.11 Register-based controller with a parity field in the micro-code

by the controller. These control signals are stored in the PM of the controller. The datapath has one
register file. The register file stores operands. (In most designs these operands are loaded from
memory. For simplicity this functionality is not included in the example.)
The register file has two read ports and one write port. For eight registers in the register file, 3 bits
of the control signal are used to select a register to be read from each read port, and 3 bits of the
control signal are used to select a register to bewritten by anALUmicro-code. The datapath based on
the result of the executed micro-code updates the Z and N bits of the ALU status register. The Z bit is
set if the result of an ALU micro-code is 0, while a negative value of the ALU result sets the N bit of

2 1 8 2 3 3 3

cond sel parity label out sel src 2 dst

src 1

22

(a)

1 2 1 8

type
sel

label

cond sel
out sel

parity

src 1 src 2 dst

2 3 3 3

12

(b)

Figure 10.12 Different fields in the micro-code of the programmable state machine design of the text
example. (a) Non-overlapping fields. (b) Overlapping fields to reduce the width of the PM