collapseos/apps/forth/main.asm
2020-03-11 17:53:27 -04:00

132 lines
3.2 KiB
NASM

; *** Const ***
; Base of the Return Stack
.equ RS_ADDR 0xf000
; Number of bytes we keep as a padding between HERE and the scratchpad
.equ PADDING 0x20
; Max length of dict entry names
.equ NAMELEN 7
; Offset of the code link relative to the beginning of the word
.equ CODELINK_OFFSET NAMELEN+3
; *** Variables ***
.equ INITIAL_SP FORTH_RAMSTART
.equ CURRENT @+2
.equ HERE @+2
; Pointer to where we currently are in the interpretation of the current line.
.equ INPUTPOS @+2
; Pointer to where compiling words should output. During interpret, it's a
; moving target in (COMPBUF). During DEFINE, it's (HERE).
.equ CMPDST @+2
; Buffer where we compile the current input line. Same size as STDIO_BUFSIZE.
.equ COMPBUF @+2
.equ FORTH_RAMEND @+0x40
; EXECUTION MODEL
; After having read a line through stdioReadLine, we want to interpret it. As
; a general rule, we go like this:
;
; 1. read single word from line
; 2. compile word to atom
; 3. execute atom
; 4. goto 1
;
; During step 3, it's possible that atom read from input, so INPUTPOS might
; have moved between 3 and 4.
;
; Because the Parameter Stack uses PS, we can't just go around calling routines:
; This messes with the PS. This is why we almost always jump (unless our call
; doesn't involve Forth words in any way).
;
; This presents a challenge for our interpret loop because step 4, "goto 1"
; isn't obvious. To be able to do that, we must push a "return routine" to the
; Return Stack before step 3.
; *** Code ***
forthMain:
; STACK OVERFLOW PROTECTION:
; To avoid having to check for stack underflow after each pop operation
; (which can end up being prohibitive in terms of costs), we give
; ourselves a nice 6 bytes buffer. 6 bytes because we seldom have words
; requiring more than 3 items from the stack. Then, at each "exit" call
; we check for stack underflow.
push af \ push af \ push af
ld (INITIAL_SP), sp
ld hl, LATEST
ld (CURRENT), hl
ld hl, FORTH_RAMEND
ld (HERE), hl
forthRdLine:
ld hl, msgOk
call printstr
call printcrlf
call stdioReadLine
ld ix, RS_ADDR-2 ; -2 because we inc-before-push
ld (INPUTPOS), hl
ld hl, COMPBUF
ld (CMPDST), hl
forthInterpret:
call readword
jr nz, .execute
call find
jr nz, .maybeNum
ex de, hl
call HLisIMMED
jr z, .immed
ex de, hl
call .writeDE
jr forthInterpret
.maybeNum:
push hl ; --> lvl 1. save string addr
call parseLiteral
pop hl ; <-- lvl 1
jr nz, .undef
; a valid number in DE!
ex de, hl
ld de, NUMBER
call .writeDE
ex de, hl ; number in DE
call .writeDE
jr forthInterpret
.undef:
; When encountering an undefined word during compilation, we spit a
; reference to litWord, followed by the null-terminated word.
; This way, if a preceding word expect a string literal, it will read it
; by calling readLIT, and if it doesn't, the routine will be
; called, triggering an abort.
ld de, LIT
call .writeDE
ld de, (CMPDST)
call strcpyM
ld (CMPDST), de
jr forthInterpret
.immed:
push hl ; --> lvl 1
ld hl, .retRef
call pushRS
pop iy ; <-- lvl 1
jp executeCodeLink
.execute:
ld de, QUIT
call .writeDE
ld iy, COMPBUF
jp compiledWord
.writeDE:
push hl
ld hl, (CMPDST)
ld (hl), e
inc hl
ld (hl), d
inc hl
ld (CMPDST), hl
pop hl
ret
.retRef:
.dw $+2
.dw $+2
call popRS
jr forthInterpret
msgOk:
.db " ok", 0