collapseos/forth/main.asm

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NASM
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; *** 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
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; Offset of the code link relative to the beginning of the word
.equ CODELINK_OFFSET NAMELEN+3
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; Flags for the "flag field" of the word structure
; IMMEDIATE word
.equ FLAG_IMMED 0
; This wordref is not a regular word (it's not preceeded by a name). It's one
; of the NUMBER, LIT, BRANCH etc. entities.
.equ FLAG_UNWORD 1
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; *** Variables ***
.equ INITIAL_SP FORTH_RAMSTART
; wordref of the last entry of the dict.
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.equ CURRENT @+2
; Pointer to the next free byte in dict.
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.equ HERE @+2
; Interpreter pointer. See Execution model comment below.
.equ IP @+2
; Pointer to where we currently are in the interpretation of the current line.
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.equ INPUTPOS @+2
; Pointer to the system's number parsing function. It points to then entry that
; had the "(parse)" name at startup. During stage0, it's out builtin PARSE,
; but at stage1, it becomes "(parse)" from core.fs. It can also be changed at
; runtime.
.equ PARSEPTR @+2
.equ FORTH_RAMEND @+2
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; (HERE) usually starts at RAMEND, but in certain situations, such as in stage0,
; (HERE) will begin at a strategic place.
.equ HERE_INITIAL FORTH_RAMEND
; 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. Can we find the word in dict?
; 3. If yes, execute that word, goto 1
; 4. Is it a number?
; 5. If yes, push that number to PS, goto 1
; 6. Error: undefined word.
;
; EXECUTING A WORD
;
; At it's core, executing a word is having the wordref in IY and call
; EXECUTE. Then, we let the word do its things. Some words are special,
; but most of them are of the compiledWord type, and that's their execution that
; we describe here.
;
; First of all, at all time during execution, the Interpreter Pointer (IP)
; points to the wordref we're executing next.
;
; When we execute a compiledWord, the first thing we do is push IP to the Return
; Stack (RS). Therefore, RS' top of stack will contain a wordref to execute
; next, after we EXIT.
;
; At the end of every compiledWord is an EXIT. This pops RS, sets IP to it, and
; continues.
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; *** Code ***
forthMain:
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; 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
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ld (INITIAL_SP), sp
; LATEST is a *indirect* label to the latest entry of the dict. See
; default at the bottom of dict.asm. This indirection allows us to
; override latest to a value set in a binary dict compiled separately,
; for example by the stage0 bin.
ld hl, LATEST
call intoHL
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ld (CURRENT), hl
ld hl, HERE_INITIAL
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ld (HERE), hl
; Set (INPUTPOS) to somewhere where there's a NULL so we consider
; ourselves EOL.
ld (INPUTPOS), hl
xor a
ld (hl), a
; Set up PARSEPTR
ld hl, PARSE-CODELINK_OFFSET
call find
ld (PARSEPTR), de
forthRdLine:
ld hl, msgOk
call printstr
forthRdLineNoOk:
; Setup return stack. After INTERPRET, we run forthExecLine
ld ix, RS_ADDR
ld hl, MAINLOOP
push hl
jp EXECUTE+2
.db 0b10 ; UNWORD
INTERPRET:
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.dw compiledWord
.dw FIND_
.dw CSKIP
.dw .maybeNum
; It's a word, execute it
.dw EXECUTE
.dw EXIT
.maybeNum:
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.dw compiledWord
.dw PARSEI
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.dw R2P ; exit INTERPRET
.dw DROP
.dw EXIT
.db 0b10 ; UNWORD
MAINLOOP:
.dw compiledWord
.dw INTERPRET
.dw INP
.dw FETCH
.dw CFETCH
.dw CSKIP
.dw QUIT
.dw MAINLOOP
msgOk:
.db " ok", 0