From b6f008da284871b73bcc3525ad9214ed09fc6121 Mon Sep 17 00:00:00 2001
From: Nathan Reiner <nathan@nathanreiner.xyz>
Date: Fri, 27 Dec 2024 13:44:52 +0100
Subject: implement loop operators

---
 bfc.S | 50 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 8 deletions(-)

(limited to 'bfc.S')

diff --git a/bfc.S b/bfc.S
index 04c3b39..134d2d1 100644
--- a/bfc.S
+++ b/bfc.S
@@ -28,15 +28,11 @@ t_move_head_left:
 t_move_head_left_end:
 
 t_increase_cell:
-	mov (%rbx), %al
-	inc %al
-	mov %al, (%rbx)
+	incb (%rbx)
 t_increase_cell_end:
 
 t_decrease_cell:
-	mov (%rbx), %al
-	dec %al
-	mov %al, (%rbx)
+	decb (%rbx)
 t_decrease_cell_end:
 
 t_write_cell:
@@ -55,6 +51,18 @@ t_read_cell:
 	syscall
 t_read_cell_end:
 
+t_check_zero:
+	mov (%rbx), %rax
+	cmp $0, %rax
+	.byte 0xf, 0x84  # `je` instruction
+	.int 0x0         # relative offset
+t_check_zero_end:
+
+t_jump_back:
+	.byte 0xe9 # `jmp` instruction
+	.int 0x0         # absolute offset
+t_jump_back_end:
+
 .section constants, "a"
 
 c_elf_header:
@@ -230,11 +238,31 @@ l_compile_dec:
 	call f_write_to_buffer
 	jmp l_read_loop
 l_compile_loop_start:
-	jmp l_read_loop
+	mov $v_buffer_cursor, %rbx
+	push (%rbx)
 
-l_compile_loop_end:
+	mov $t_check_zero, %rdi
+	mov $t_check_zero_end, %rsi
+	call f_write_to_buffer
 	jmp l_read_loop
+l_compile_loop_end:
+	pop %r15
 
+	mov $t_jump_back, %rdi
+	mov $t_jump_back_end, %rsi
+	call f_write_to_buffer
+
+	mov $v_buffer_cursor, %rbx
+	mov (%rbx), %rax
+
+	mov %r15, %rbx
+	sub %rax, %r15
+	mov %r15d, -4(%rax)
+	neg %r15
+	sub $13, %r15
+	mov %r15d, 9(%rbx)
+
+	jmp l_read_loop
 l_compile_write:
 	mov $t_write_cell, %rdi
 	mov $t_write_cell_end, %rsi
@@ -385,6 +413,12 @@ f_open_files:
 	syscall
 	mov $v_output_fd, %rbx
 	mov %eax, (%rbx)
+
+	mov %eax, %edi
+	mov $0, %rsi
+	mov $77, %rax
+	syscall
+
 	ret
 
 
-- 
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