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
+
+;; 0BRANCH is the fundamental mechanism for branching. If the top of the stack
+;; is zero, we jump by the given offset. 0BRANCH is given the offset as an
+;; integer after the word.
+ZBRANCH:
+ dq .start
+.start:
+ ;; Compare top of stack to see if we should branch
+ pop rax
+ cmp rax, 0
+ jnz .dont_branch
+.do_branch:
+ 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
+.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:
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 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: