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shave some bytes #5

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85 changes: 40 additions & 45 deletions sectorforth.asm
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Original file line number Diff line number Diff line change
Expand Up @@ -13,10 +13,7 @@
; The SP register is used as the data stack pointer, and the BP
; register acts as the return stack pointer.
;
; The minimum CPU required to run sectorforth is the 386, to use
; the SETNZ instruction.
bits 16
cpu 386

; Set CS to a known value by performing a far jump. Memory up to
; 0x0500 is used by the BIOS. Setting the segment to 0x0500 gives
Expand All @@ -36,7 +33,7 @@ TIB equ 0x0000 ; terminal input buffer (TIB)
STATE equ 0x1000 ; current state (0=interpret, 1=compile)
TOIN equ 0x1002 ; current read offset into TIB (>IN)
RP0 equ 0x76fe ; bottom of return stack
SP0 equ 0xfffe ; bottom of data stack
SP0 equ TIB ; bottom of data stack

; Each dictionary entry is laid out in memory as such:
;
Expand Down Expand Up @@ -106,10 +103,8 @@ word_%2:

defword "0=",ZEROEQUALS
pop ax
test ax,ax
setnz al ; AL=0 if ZF=1, else AL=1
dec ax ; AL=ff if AL=0, else AL=0
cbw ; AH=AL
cmp ax,1 ; C=1 if AX=0
sbb ax,ax ; AX-AX-C, 0->FFFF, else 0
push ax
NEXT

Expand Down Expand Up @@ -195,9 +190,12 @@ LATEST: dw word_SEMICOLON ; initialized to last word in dictionary
call token ; parse word from input
push si
mov si,di ; set parsed word as string copy source
mov di,[HERE] ; set current value of HERE as destination
mov ax,[LATEST] ; get pointer to latest defined word
mov [LATEST],di ; update LATEST to new word being defined
mov bx,HERE
mov di,[bx] ; set current value of HERE as destination
mov ax,di
xchg [bx+LATEST-HERE],ax
; get pointer to latest defined word, and
; update LATEST to new word being defined
stosw ; link pointer
mov al,cl
or al,F_HIDDEN ; hide new word while it's being defined
Expand All @@ -207,7 +205,7 @@ LATEST: dw word_SEMICOLON ; initialized to last word in dictionary
stosw ; compile near jump, absolute indirect...
mov ax,DOCOL.addr
stosw ; ...to DOCOL
mov [HERE],di ; update HERE to next free position
mov [bx],di ; update HERE to next free position
mov byte [STATE],1 ; switch to compilation state
pop si
NEXT
Expand All @@ -219,7 +217,7 @@ LATEST: dw word_SEMICOLON ; initialized to last word in dictionary
; pointer), and point it to the sequence of addresses that makes
; up a word's body.
;
; DOCOL advances AX 4 bytes, and then moves that value to SI. When
; DOCOL moves AX to SI, and then advances SI by 4 bytes. When
; DOCOL is jumped to, AX points to the code field of the word
; about to be executed. The 4 bytes being skipped are the actual
; jump instruction to DOCOL itself, inserted by the colon compiler
Expand All @@ -228,8 +226,9 @@ DOCOL:
xchg sp,bp ; swap SP and BP, SP controls return stack
push si ; push current "instruction pointer"
xchg sp,bp ; restore SP and BP
add ax,4 ; skip word's code field
mov si,ax ; point "instruction pointer" to word body
xchg si,ax ; point "instruction pointer" to word body
lodsw
lodsw ; skip word's code field
NEXT ; start executing the word

; The jump instruction inserted by the compiler is an indirect
Expand All @@ -252,6 +251,14 @@ compile:
mov [HERE],di ; advance HERE to next cell
NEXT

; Display a red '!!' to let the user know an error happened and the
; interpreter is being reset
error:
mov ax,0x0921 ; write '!'
mov bl,0x04 ; black background (BH=0 already), red text
mov cl,2 ; twice (CH should be 0 already)
int 0x10

; Execution starts here.
start:
cld ; clear direction flag
Expand All @@ -264,26 +271,15 @@ start:
pop es
pop ss

; Skip error signaling on initialization
jmp init

; Display a red '!!' to let the user know an error happened and the
; interpreter is being reset
error:
mov ax,0x0921 ; write '!'
mov bx,0x0004 ; black background, red text
mov cx,2 ; twice
int 0x10

; Initialize stack pointers, state, and terminal input buffer.
init:
xor sp,sp ; mov sp,SP0 / data stack pointer
mov bp,RP0 ; BP is the return stack pointer
mov sp,SP0 ; SP is the data stack pointer

; Fill TIB with zeros, and set STATE and >IN to 0
mov al,0
mov cx,STATE+4
mov di,TIB
xor di,di ; mov di,TIB
rep stosb

; Enter the interpreter loop.
Expand All @@ -298,28 +294,23 @@ init:
; input buffer, and the interpreter goes into interpretation mode.
interpreter:
call token ; parse word from input
mov bx,[LATEST] ; start searching for it in the dictionary
.1: test bx,bx ; zero?
mov bx,LATEST ; start searching for it in the dictionary
.1: mov bx,[bx]
test bx,bx ; zero?
jz error ; not found, reset interpreter state
mov si,bx
lodsw ; skip link
lea si,[bx+2] ; skip link
lodsb ; read flags+length
mov dl,al ; save those for later use
test al,F_HIDDEN ; entry hidden?
jnz .2 ; if so, skip it
and al,LENMASK ; mask out flags
jnz .1 ; if so, skip it
mov dl,al ; save those for later use
and ax,LENMASK ; mask out flags
cmp al,cl ; same length?
jne .2 ; if not, skip entry
push cx
jne .1 ; if not, skip entry
push di
repe cmpsb ; compare strings
pop di
pop cx
je .3 ; if equal, search is over
.2: mov bx,[bx] ; skip to next entry
jmp .1 ; try again
.3: mov ax,si ; after comparison, SI points to code field
mov si,.loop ; set SI so NEXT loops back to interpreter
xchg cx,ax ; restore length
jne .1 ; try again
; Decide whether to interpret or compile the word. The IMMEDIATE
; flag is located in the most significant bit of the flags+length
; byte. STATE can only be 0 or 1. When ORing those two, these are
Expand All @@ -337,6 +328,10 @@ interpreter:
and dl,F_IMMEDIATE ; isolate IMMEDIATE flag
or dl,[STATE] ; OR with state
dec dl ; decrement
xchg si,ax ; after comparison, SI points to code field
; move it into AX
mov si,.loop ; set SI so NEXT loops back to interpreter
; Z flag is unaffected by XCHG and MOV
jz compile ; if result is zero, compile
jmp ax ; otherwise, interpret (execute) the word
.loop: dw interpreter
Expand Down Expand Up @@ -372,7 +367,7 @@ token:
call writechar ; CR
mov al,10
call writechar ; LF
mov di,TIB ; read into TIB
xor di,di ; mov di,TIB / read into TIB
.1: mov ah,0 ; wait until a key is pressed
int 0x16
cmp al,13 ; return pressed?
Expand All @@ -382,7 +377,7 @@ token:
call writechar ; otherwise, write character to screen
stosb ; store character in TIB
jmp .1 ; keep reading
.2: cmp di,TIB ; start of TIB?
.2: test di,di ; cmp di,TIB / start of TIB?
je .1 ; if so, there's nothing to erase
dec di ; erase character in TIB
call writechar ; move cursor back one character
Expand Down