From fdf65bafcc4d2040a9432e50cfec5b6d525c70c8 Mon Sep 17 00:00:00 2001
From: Jacques Comeaux <jacquesrcomeaux@gmail.com>
Date: Sun, 18 Sep 2022 19:20:42 -0500
Subject: Add S-Exp parser

---
 read.s | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)
 create mode 100644 read.s

(limited to 'read.s')

diff --git a/read.s b/read.s
new file mode 100644
index 0000000..7a5975d
--- /dev/null
+++ b/read.s
@@ -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
-- 
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