X-Git-Url: https://git.rrq.au/?a=blobdiff_plain;f=main.asm;h=f665f952ba5c44ffa02204c2528ee5f4ca090daa;hb=0e388e347f54e828df6b9b227c4009584b2173d7;hp=acb119a986410f4183b4e999382d6335f36fa9a4;hpb=69a2f0a24cc7698bc49648d7419c3292140fc869;p=rrq%2Fjonasforth.git

diff --git a/main.asm b/main.asm
index acb119a..f665f95 100644
--- a/main.asm
+++ b/main.asm
@@ -1,25 +1,242 @@
 format ELF64 executable
 
-struc with_length string& {
-    . db string
-    .length = $ - .
+;; The code in this macro is placed at the end of each Forth word. When we are
+;; executing a definition, this code is what causes execution to resume at the
+;; next word in that definition.
+macro next {
+  ;; RSI points to the address of the definition of the next word to execute.
+  lodsq                   ; Load value at RSI into RAX and increment RSI
+  ;; Now RAX contains the location of the next word to execute. The first 8
+  ;; bytes of this word is the address of the codeword, which is what we want
+  ;; to execute.
+  jmp qword [rax]         ; Jump to the codeword of the current word
 }
 
-macro write_stdout string_label {
-    mov rax, 1
-    mov rdi, 1
-    mov rsi, string_label
-    mov rdx, string_label#.length
-    syscall
+;; pushr and popr work on the return stack, whose location is stored in the
+;; register RBP.
+macro pushr x {
+  sub rbp, 8
+  mov qword [rbp], x
+}
+macro popr x {
+  mov x, [rbp]
+  add rbp, 8
 }
 
 segment readable executable
 
-start:
-    write_stdout message
+main:
+  cld                        ; Clear direction flag so LODSQ does the right thing.
+  mov rbp, return_stack_top  ; Initialize return stack
+
+  mov rsi, program
+  next
+
+program: dq MAIN
+
+;; The codeword is the code that will be executed at the beginning of a forth
+;; word. It needs to save the old RSI and update it to point to the next word to
+;; execute.
+docol:
+  pushr rsi            ; Save old value of RSI on return stack; we will continue execution there after we are done executing this word
+  lea rsi, [rax + 8]   ; RAX currently points to the address of the codeword, so we want to continue at RAX+8
+  next                 ; Execute word pointed to by RSI
+
+;; This word is called at the end of a Forth definition. It just needs to
+;; restore the old value of RSI (saved by 'docol') and resume execution.
+EXIT:
+  dq .start
+.start:
+  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:
+  pushr rsi
+  pushr rax
+  mov rax, 1
+  mov rdi, 1
+  lea rsi, [rsp]
+  mov rdx, 1
+  syscall
+  add rsp, 8
+  popr rax
+  popr rsi
+  next
+
+;; Prints a newline to standard output.
+NEWLINE:
+  dq docol
+  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:
+  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 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
 
-    jmp $
+you_typed_string db 'You typed: '
+.length = $ - you_typed_string
 
-segment readable
+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 ?
 
-message with_length 'Hello, world!',$A
+;; Return stack
+rq $2000
+return_stack_top: