Return Stack Lecture 8.4 A VHDL Forth Core for FPGAs: Sect. 5

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

A 32 x 16 Stack Module

The Return Stack

The WC16C WHYP Core

FC16 Return Stack 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';

FC16 Return Stack 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';

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

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 HEX : 1ms_Delay( -- ) 30D1 FOR NEXT ; :.5sec_Delay ( -- ) 1F4 FOR 1ms_Delay NEXT ; : MAIN ( -- ) BEGIN FOR ROM Listing