Return Stack Lecture L7.3

A 32 x 16 Stack Same as used in the Data Stack in Lab 7

A 32 x 16 Stack Module

The Return Stack

The WC16 WHYP Core

CodeNameFunction 0030>R “To-R” Pop T and push it on return stack. 0031R> “R-from” Pop return stack R and push it into T. “R-fetch” Copy R to T and push data stack 0033R>DROP “R-from-drop” Pop return stack R and throw it away 0039LD! “LD store” Store 8-bits of T into the 8-bit LD register Return Stack and I/O Instructions

when tor => tload <= '1'; nload <= '1'; tsel <= "111"; nsel <= "01"; dpop <= '1'; rload <= '1'; rpush <= '1'; rinsel <= '1'; “To-R” Pop T and push it on return stack. >R

“R-from” Pop return stack R and push it into T. R> when rfrom => tload <= '1'; nload <= '1'; tsel <= "011"; dpush <= '1'; rsel <= '1'; rload <= '1'; rpop <= '1';

“R-fetch” Copy R to T and push data stack when rfetch => tload <= '1'; nload <= '1'; tsel <= "011"; dpush <= '1';

“R-from-drop” Pop return stack R and throw it away R>DROP when rfromdrop => rsel <= '1'; rload <= '1'; rpop <= '1';

CodeNameFunction 0030>R “To-R” Pop T and push it on return stack. 0031R> “R-from” Pop return stack R and push it into T. “R-fetch” Copy R to T and push register stack 0033R>DROP “R-from-drop” Pop return stack R and throw it away 0039LD! “LD store” Store 8-bits of T into the 8-bit LD register Return Stack and I/O Instructions

when ldstore => ldload <= '1'; tload <= '1'; nload <= '1'; tsel <= "111"; nsel <= "01"; dpop <= '1'; “LD store” Store 8-bits of T into the 8-bit LD register and pop data stack LD! ldreg LD(7:0) clr clk ldload

CodeNameFunction 0103DRJNE Decrement R and jump if R is not zero 0104CALL Call subroutine 0105RET Subroutine return Transfer Instructions

>R Decrement top of return stack and branch back to if not equal to zero. Therefore, are executed n times. FOR…NEXT Loop n FOR NEXT drjne

Decrement R and jump if R is not zero DRJNE when drjne => rdec <= not r1; pload <= not r1; psel <= '0'; pinc <= r1; rsel <= r1; rload <= r1; rpop <= r1; -- r1 <= '1' if R-1 is all zeros r1 := '0'; for i in width-1 downto 1 loop r1 := r1 or R(i); end loop; r1 := (not r1) and R(0);

The Return Stack

Call subroutine Jump to address M and push address of next instruction (P + 1) on return stack CALL when call => pload <= '1'; rload <= '1'; rpush <= '1'; call, X”1234”, In ROM:

Subroutine return Jump to address on top of the return stack, and pop the return stack RET when ret => psel <= '1'; pload <= '1'; rsel <= '1'; rload <= '1'; rpop <= '1';