Return Stack Lab 8

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 WC16C WHYP Core

Return Stack and I/O Instructions Code Name Function 0030 >R “To-R” Pop T and push it on return stack. 0031 R> “R-from” Pop return stack R and push it into T. 0032 R@ “R-fetch” Copy R to T and push register stack 0033 R>DROP “R-from-drop” Pop return stack R and throw it away 0039 LD! “LD store” Store 8-bits of T into the 8-bit LD register

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

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

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

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

Return Stack and I/O Instructions Code Name Function 0030 >R “To-R” Pop T and push it on return stack. 0031 R> “R-from” Pop return stack R and push it into T. 0032 R@ “R-fetch” Copy R to T and push register stack 0033 R>DROP “R-from-drop” Pop return stack R and throw it away 0039 LD! “LD store” Store 8-bits of T into the 8-bit LD register

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

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

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

DRJNE Decrement R and jump if R is not zero -- 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); when drjne => rdec <= not r1; pload <= not r1; psel <= '0'; pinc <= r1; rsel <= r1; rload <= r1; rpop <= r1;

The Return Stack

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

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

\ led8.whp HEX   : 1ms_Delay ( -- ) 30D1 FOR NEXT ; : .5sec_Delay ( -- ) 1F4 FOR 1ms_Delay NEXT ; : MAIN ( -- ) BEGIN 8000 8 FOR DUP LD! DUP DIG! .5sec_Delay 2/ NEXT U2/ DROP AGAIN ;

ROM Listing constant rom: rom_array := ( JMP, --0 X"0010", --1 LIT, --2 X"30D1", --3 tor, --4 drjne, --5 X"0005", --6 RET, --7 LIT, --8 X"03E8", --9 tor, --a CALL, --b X"0002", --c drjne, --d X"000b", --e RET, --f LIT, --10 X"8000", --11 LIT, --12 X"0008", --13 tor, --14 ROM Listing HEX   : 1ms_Delay ( -- ) 30D1 FOR NEXT ; : .5sec_Delay ( -- ) 1F4 FOR 1ms_Delay NEXT ; : MAIN ( -- ) BEGIN 8000 8 FOR