1. Return Stack
Lab 8

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

3. A 32 x 16 Stack Module

4. The Return Stack

5. The WC16C
WHYP Core

6. 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-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

7. >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.

8. 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.

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

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

11. 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-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

12. 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!

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

14. 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.

15. 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;

16. The Return Stack

17. 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';

18. 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';

20. \ 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 ;

21. 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,
X"8000", --11
LIT,
X"0008", --13
tor,
ROM Listing
HEX
: 1ms_Delay ( -- )
30D1 FOR NEXT ;
: .5sec_Delay ( -- )
1F4 FOR 1ms_Delay NEXT ;
: MAIN ( -- )
BEGIN
8000
8 FOR