From 8a168f2be5d90f972975abf0b40145a75c0231b7 Mon Sep 17 00:00:00 2001
From: dzwdz
Date: Wed, 21 Jul 2021 21:35:20 +0200
Subject: syscall parameter & return value passing

Sadly, sysenter on i386 limits me to only 4 arguments (so, 1 for the
syscall id + 3 real args). If that turns out to be an issue I'll either
just switch to interrupts, or switch to x64.
---
 src/init/main.c    | 6 +++---
 src/init/syscall.s | 4 ++++
 2 files changed, 7 insertions(+), 3 deletions(-)

(limited to 'src/init')

diff --git a/src/init/main.c b/src/init/main.c
index cb3fa1d..6c4ccc3 100644
--- a/src/init/main.c
+++ b/src/init/main.c
@@ -1,15 +1,15 @@
 #include <stdint.h>
 
-void _syscall();
+int _syscall(int, int, int, int);
 
 int main() {
-	_syscall();
+	uint8_t color = _syscall(1, 2, 3, 4);
 
 	// change the colors of VGA text
 	// doesn't require a lot of code, but still shows that it's working
 	uint8_t *vga = (void*) 0xB8000;
 	for (int i = 0; i < 80 * 25; i++)
-		vga[(i << 1) + 1] = 0x4e;
+		vga[(i << 1) + 1] = color;
 
 	// try to mess with kernel memory
 	uint8_t *kernel = (void*) 0x100000;
diff --git a/src/init/syscall.s b/src/init/syscall.s
index ebd9d0e..86d3523 100644
--- a/src/init/syscall.s
+++ b/src/init/syscall.s
@@ -2,8 +2,12 @@
 .global _syscall
 .type _syscall, @function
 _syscall:
+	mov 4(%esp), %eax
+	mov 8(%esp), %ebx
 	mov %esp, %ecx
 	mov $_syscall_ret, %edx
+	mov 12(%esp), %esi
+	mov 16(%esp), %edi
 	sysenter
 _syscall_ret:
 	ret
-- 
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