Also implement BRANCH

[rrq/jonasforth.git] / main.asm

diff --git a/main.asm b/main.asm
index 0a8ff4cf256dcd602e3c8c59b3bde3418422343b..f665f952ba5c44ffa02204c2528ee5f4ca090daa 100644 (file)
--- a/main.asm
+++ b/main.asm
@@ -50,6 +50,38 @@ EXIT:
   popr rsi
   next
 
+;; LIT is a special word that reads the next "word pointer" and causes it to be
+;; placed on the stack rather than executed.
+LIT:
+  dq .start
+.start:
+  lodsq
+  push rax
+  next
+
+;; BRANCH is the fundamental mechanism for branching. BRANCH reads the next word
+;; as a signed integer literal and jumps by that offset.
+BRANCH:
+  dq .start
+.start:
+  add rsi, [rsi] ; [RSI], which is the next word, contains the offset; we add this to the instruction pointer.
+  next           ; Then, we can just continue execution as normal
+
+;; 0BRANCH is like BRANCH, but it jumps only if the top of the stack is zero.
+ZBRANCH:
+  dq .start
+.start:
+  ;; Compare top of stack to see if we should branch
+  pop rax
+  cmp rax, 0
+  jnz .dont_branch
+.do_branch:
+  jmp BRANCH.start
+.dont_branch:
+  add rsi, 8     ; We need to skip over the next word, which contains the offset.
+  next
+
+;; Expects a character on the stack and prints it to standard output.
 EMIT:
   dq .start
 .start:
@@ -65,44 +97,146 @@ EMIT:
   popr rsi
   next
 
-PUSH_HELLO_CHARS:
-  dq .start
-  .start:
-  push $A
-  push 'o'
-  push 'l'
-  push 'l'
-  push 'e'
-  push 'H'
-  next
-
-HELLO:
+;; Prints a newline to standard output.
+NEWLINE:
   dq docol
-  dq PUSH_HELLO_CHARS
-  dq EMIT
-  dq EMIT
-  dq EMIT
-  dq EMIT
-  dq EMIT
+  dq LIT, $A
   dq EMIT
   dq EXIT
 
+;; Read a word from standard input and push it onto the stack as a pointer and a
+;; size. The pointer is valid until the next call to READ_WORD.
+READ_WORD:  ; 400170
+  dq .start
+.start:
+  mov [.rsi], rsi
+  mov [.rax], rax
+
+.skip_whitespace:
+  ;; Read characters into .char_buffer until one of them is not whitespace.
+  mov rax, 0
+  mov rdi, 0
+  mov rsi, .char_buffer
+  mov rdx, 1
+  syscall
+
+  cmp [.char_buffer], ' '
+  je .skip_whitespace
+  cmp [.char_buffer], $A
+  je .skip_whitespace
+
+.alpha:
+  ;; We got a character that wasn't whitespace. Now read the actual word.
+  mov [.length], 0
+
+.read_alpha:
+  mov al, [.char_buffer]
+  movzx rbx, [.length]
+  mov rsi, .buffer
+  add rsi, rbx
+  mov [rsi], al
+  inc [.length]
+
+  mov rax, 0
+  mov rdi, 0
+  mov rsi, .char_buffer
+  mov rdx, 1
+  syscall
+
+  cmp [.char_buffer], ' '
+  je .end
+  cmp [.char_buffer], $A
+  jne .read_alpha
+
+.end:
+  push .buffer
+  movzx rax, [.length]
+  push rax
+
+  mov rsi, [.rsi]
+  mov rax, [.rax]
+
+  next
+
+;; Takes a string (in the form of a pointer and a length on the stack) and
+;; prints it to standard output.
+TELL:
+  dq .start
+.start:
+  mov rbx, rsi
+  mov rcx, rax
+
+  mov rax, 1
+  mov rdi, 1
+  pop rdx     ; Length
+  pop rsi     ; Buffer
+  syscall
+
+  mov rax, rcx
+  mov rsi, rbx
+  next
+
+;; Exit the program cleanly.
 TERMINATE:
   dq .start
-  .start:
+.start:
   mov rax, $3C
   mov rdi, 0
   syscall
 
+PUSH_HELLO_CHARS:
+  dq docol
+  dq LIT, $A
+  dq LIT, 'o'
+  dq LIT, 'l'
+  dq LIT, 'l'
+  dq LIT, 'e'
+  dq LIT, 'H'
+  dq EXIT
+
+PUSH_YOU_TYPED:
+  dq .start
+.start:
+  push you_typed_string
+  push you_typed_string.length
+  next
+
+HELLO:
+  dq docol
+  dq LIT, 'H', EMIT
+  dq LIT, 'e', EMIT
+  dq LIT, 'l', EMIT
+  dq LIT, 'l', EMIT
+  dq LIT, 'o', EMIT
+  dq LIT, '!', EMIT
+  dq NEWLINE
+  dq EXIT
+
 MAIN:
   dq docol
   dq HELLO
-  dq HELLO
+  dq READ_WORD
+  dq LIT, you_typed_string
+  dq LIT, you_typed_string.length
+  dq TELL
+  dq TELL
+  dq NEWLINE
+  dq BRANCH, -72
   dq HELLO
   dq TERMINATE
 
 segment readable writable
 
+you_typed_string db 'You typed: '
+.length = $ - you_typed_string
+
+READ_WORD.rsi dq ?
+READ_WORD.rax dq ?
+READ_WORD.max_size = $FF
+READ_WORD.buffer rb READ_WORD.max_size
+READ_WORD.length db ?
+READ_WORD.char_buffer db ?
+
 ;; Return stack
 rq $2000
 return_stack_top: