mirror of https://github.com/hsoft/collapseos.git
135 lines
3.7 KiB
NASM
135 lines
3.7 KiB
NASM
; *** Const ***
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; Base of the Return Stack
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.equ RS_ADDR 0xf000
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; Number of bytes we keep as a padding between HERE and the scratchpad
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.equ PADDING 0x20
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; Max length of dict entry names
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.equ NAMELEN 7
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; Offset of the code link relative to the beginning of the word
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.equ CODELINK_OFFSET NAMELEN+3
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; Flags for the "flag field" of the word structure
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; IMMEDIATE word
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.equ FLAG_IMMED 0
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; This wordref is not a regular word (it's not preceeded by a name). It's one
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; of the NUMBER, LIT, BRANCH etc. entities.
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.equ FLAG_UNWORD 1
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; *** Variables ***
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.equ INITIAL_SP FORTH_RAMSTART
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; wordref of the last entry of the dict.
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.equ CURRENT @+2
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; Pointer to the next free byte in dict.
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.equ HERE @+2
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; Interpreter pointer. See Execution model comment below.
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.equ IP @+2
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; Pointer to where we currently are in the interpretation of the current line.
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.equ INPUTPOS @+2
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; Pointer to the system's number parsing function. It points to then entry that
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; had the "(parse)" name at startup. During stage0, it's out builtin PARSE,
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; but at stage1, it becomes "(parse)" from core.fs. It can also be changed at
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; runtime.
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.equ PARSEPTR @+2
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.equ FORTH_RAMEND @+2
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; (HERE) usually starts at RAMEND, but in certain situations, such as in stage0,
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; (HERE) will begin at a strategic place.
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.equ HERE_INITIAL FORTH_RAMEND
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; EXECUTION MODEL
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; After having read a line through stdioReadLine, we want to interpret it. As
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; a general rule, we go like this:
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;
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; 1. read single word from line
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; 2. Can we find the word in dict?
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; 3. If yes, execute that word, goto 1
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; 4. Is it a number?
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; 5. If yes, push that number to PS, goto 1
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; 6. Error: undefined word.
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;
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; EXECUTING A WORD
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;
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; At it's core, executing a word is having the wordref in IY and call
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; EXECUTE. Then, we let the word do its things. Some words are special,
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; but most of them are of the compiledWord type, and that's their execution that
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; we describe here.
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;
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; First of all, at all time during execution, the Interpreter Pointer (IP)
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; points to the wordref we're executing next.
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;
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; When we execute a compiledWord, the first thing we do is push IP to the Return
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; Stack (RS). Therefore, RS' top of stack will contain a wordref to execute
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; next, after we EXIT.
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;
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; At the end of every compiledWord is an EXIT. This pops RS, sets IP to it, and
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; continues.
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; *** Code ***
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forthMain:
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; STACK OVERFLOW PROTECTION:
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; To avoid having to check for stack underflow after each pop operation
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; (which can end up being prohibitive in terms of costs), we give
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; ourselves a nice 6 bytes buffer. 6 bytes because we seldom have words
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; requiring more than 3 items from the stack. Then, at each "exit" call
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; we check for stack underflow.
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push af \ push af \ push af
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ld (INITIAL_SP), sp
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; LATEST is a *indirect* label to the latest entry of the dict. See
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; default at the bottom of dict.asm. This indirection allows us to
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; override latest to a value set in a binary dict compiled separately,
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; for example by the stage0 bin.
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ld hl, LATEST
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call intoHL
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ld (CURRENT), hl
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ld hl, HERE_INITIAL
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ld (HERE), hl
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; Set (INPUTPOS) to somewhere where there's a NULL so we consider
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; ourselves EOL.
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ld (INPUTPOS), hl
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xor a
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ld (hl), a
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; Set up PARSEPTR
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ld hl, PARSE-CODELINK_OFFSET
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call find
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ld (PARSEPTR), de
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forthRdLine:
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ld hl, msgOk
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call printstr
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forthRdLineNoOk:
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; Setup return stack. After INTERPRET, we run forthExecLine
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ld ix, RS_ADDR
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ld hl, MAINLOOP
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push hl
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jp EXECUTE+2
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.db 0b10 ; UNWORD
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INTERPRET:
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.dw compiledWord
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.dw FIND_
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.dw CSKIP
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.dw .maybeNum
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; It's a word, execute it
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.dw EXECUTE
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.dw EXIT
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.maybeNum:
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.dw compiledWord
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.dw PARSEI
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.dw R2P ; exit INTERPRET
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.dw DROP
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.dw EXIT
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.db 0b10 ; UNWORD
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MAINLOOP:
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.dw compiledWord
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.dw INTERPRET
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.dw INP
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.dw FETCH
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.dw CFETCH
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.dw CSKIP
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.dw QUIT
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.dw MAINLOOP
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msgOk:
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.db " ok", 0
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