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Return Stack

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';

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Return Stack

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

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

  3. A 32 x 16 Stack Module

  4. The Return Stack

  5. The WC16C WHYP Core

  6. FC16 Return Stack Instructions

  7. 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.

  8. 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.

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

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

  11. FC16 Return Stack Instructions

  12. 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.

  13. 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;

  14. The Return Stack

  15. 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'; In ROM: call, X”1234”, --12 --14

  16. 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';

  17. \ 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 8 FOR U2/ DUP LD! DUP DIG! .5sec_Delay NEXT DROP AGAIN ;

  18. 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

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