diff options
Diffstat (limited to 'read.s')
| -rw-r--r-- | read.s | 64 |
1 files changed, 64 insertions, 0 deletions
@@ -0,0 +1,64 @@ +read: +MOV #7000, R5 ; move read buffer address into R5 +; populate read buffer with console input until enter key is pressed +; filter out bad characters here. +; increment R5 along the way. +; and start with null byte. +; what we are left with is just (, ), A-Z, and space +JSR PC, #parse_sexp +JMP #eval + +; R4 = sexps + +parse_sexp: +JSR PC, #get_next ; get a character +CMPB R0, ")" +BNE if_atom ; if atom get the atom +; otherwise (if list) +; get the list +if_list: +MOV "NIL", R4 ; sexps <- empty list +loop: +JSR PC, #get_next +CMPB R0, "(" +BEQ done ; if "(" return the accumulated list +; if anything else: +MOV R4, -(SP) ; push sexps +JSR PC, #parse_sexp ; arg1 <- parse sexp +MOV (SP)+, R1 ; arg2 <- pop accum +JSR PC, #cons ; cons result onto accum +MOV R0, R4 ; sexps <- result +BR -??? ; continue recognizing list +done: +MOV R4, R0 ; result <- sexps +RTS PC +if_atom: +MOVB "\0", -(SP) ; push null byte to stack +loop: +MOVB R0, -(SP) ; push character to stack ; +MOVB -(R5), R0 ; get another character +CMPB R0, "A" ; +BLO done ; done if lower than A +CMPB R0, "Z" ; +BHI done ; done if higher than Z +BR -??? ; continue getting characters +done: +MOVB R0, (R5)+ ; put the non-letter back +MOV @#10000, R2 ; get free pointer +MOV R2, R0 ; result <- address of new atom +MOV 3(R2), (R2)+ ; allocate atom tag and increment free pointer +MOVB (SP)+, (R2)+ ; pop a char off the stack and into the heap ; increment the free pointer (by 1) +BNE -4 ; if it wasn't null keep going +TST (R2)+ ; align free pointer +MOV R2, @#10000 ; store new free pointer +RTS PC + +get_next: +loop: +MOVB -(R5), R0 +BEQ bad ; if null byte, no more input, very bad +CMPB R0, " " ; check if space +BEQ loop ; skip space +RTS PC ; return the lexically-valid character +bad: +BR -2 ; infinite loop |