collapseos/apps/forth/stack.asm
2020-03-17 17:29:03 -04:00

67 lines
1.4 KiB
NASM

; The Parameter stack (PS) is maintained by SP and the Return stack (RS) is
; maintained by IX. This allows us to generally use push and pop freely because
; PS is the most frequently used. However, this causes a problem with routine
; calls: because in Forth, the stack isn't balanced within each call, our return
; offset, when placed by a CALL, messes everything up. This is one of the
; reasons why we need stack management routines below. IX always points to RS'
; Top Of Stack (TOS)
;
; This return stack contain "Interpreter pointers", that is a pointer to the
; address of a word, as seen in a compiled list of words.
; Push value HL to RS
pushRS:
inc ix
inc ix
ld (ix), l
ld (ix+1), h
ret
; Pop RS' TOS to HL
popRS:
ld l, (ix)
ld h, (ix+1)
dec ix
dec ix
ret
popRSIP:
call popRS
ld (IP), hl
ret
; Skip the next two bytes in RS' TOS
skipRS:
push hl
ld l, (ix)
ld h, (ix+1)
inc hl \ inc hl
ld (ix), l
ld (ix+1), h
pop hl
ret
; Verifies that SP and RS are within bounds. If it's not, call ABORT
chkRS:
push ix \ pop hl
push de ; --> lvl 1
ld de, RS_ADDR
or a ; clear carry
sbc hl, de
pop de ; <-- lvl 1
jp c, abortUnderflow
ret
chkPS:
push hl
ld hl, (INITIAL_SP)
; We have the return address for this very call on the stack and
; protected registers. Let's compensate
dec hl \ dec hl
dec hl \ dec hl
or a ; clear carry
sbc hl, sp
pop hl
ret nc ; (INITIAL_SP) >= SP? good
jp abortUnderflow