Micro-programmed State Machines - Digital Design of Signal Processing Systems: A Practical Approach - page 437

from PC

Return Address
Stack

N

write_en

1

write_lifo_addr

current

2

call
sub

return

to

incremented next_address_mux

Stack Pointer logic

read_lifo_addr

clk

rst_n

Figure 10.16 Subroutine return address stack

A representative instruction for a loop can be written as in Figure 10.17. The instruction provides
the loop end-address and the count for the number of times the loop needs to be repeated. As counter
runs down to zero therefore while specifying the loop count one must be subtracted from the actual
count the loop needs to run. In the cycle in which the state machine executes the loop instruction, the
PC stores the address of the next instruction that also is the start-address of the loop. In the example,
the loop instruction is at address 80, PC contains the next address 81, and the loop instruction
specifies the loop end-address and the loop counter as 100 and 20, respectively. The loop machine
stores these three values in loop_start_addr_reg , loop_end_addr_reg and loop_
count_reg , respectively. The value in the PC is continuously compared with the loop end-
address. If the PC is equal to the loop end-address, the value in loop_count_reg is decremented
by 1 and is checked whether the down count is equal to zero. If it is not, the next-address logic selects

address of
instructions

Loop count

79
80
81
82

Loop end
address

repeat 20, 100

Loop start addr

Loop
body

100

Figure 10.17 Representative instruction for a loop