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
+
+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
+
+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
+
+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: