X-Git-Url: https://git.rrq.au/?a=blobdiff_plain;f=main.asm;h=c6ada7329cbd4b320947c8b925203ad448ac162d;hb=c142f4b8739ed42a9afde1c3cd69c30a1aa9c606;hp=acb119a986410f4183b4e999382d6335f36fa9a4;hpb=69a2f0a24cc7698bc49648d7419c3292140fc869;p=rrq%2Fjonasforth.git

diff --git a/main.asm b/main.asm
index acb119a..c6ada73 100644
--- a/main.asm
+++ b/main.asm
@@ -1,25 +1,108 @@
 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 {
+;; 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
+
+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
+
+EMIT:
+    dq .start
+.start:
+    pushr rsi
+    pushr rax
     mov rax, 1
     mov rdi, 1
-    mov rsi, string_label
-    mov rdx, string_label#.length
+    lea rsi, [rsp]
+    mov rdx, 1
     syscall
-}
+    add rsp, 8
+    popr rax
+    popr rsi
+    next
 
-segment readable executable
+PUSH_HELLO_CHARS:
+    dq .start
+.start:
+    push $A
+    push 'o'
+    push 'l'
+    push 'l'
+    push 'e'
+    push 'H'
+    next
+
+HELLO:
+    dq docol
+    dq PUSH_HELLO_CHARS
+    dq EMIT
+    dq EMIT
+    dq EMIT
+    dq EMIT
+    dq EMIT
+    dq EMIT
+    dq EXIT
 
-start:
-    write_stdout message
+TERMINATE:
+    dq .start
+.start:
+    mov rax, $3C
+    mov rdi, 0
+    syscall
 
-    jmp $
+MAIN:
+    dq docol
+    dq HELLO
+    dq HELLO
+    dq HELLO
+    dq TERMINATE
 
-segment readable
+segment readable writable
 
-message with_length 'Hello, world!',$A
+;; Return stack
+rq $2000
+return_stack_top: