From 6982057b61e939fa14418356969753a3c33ebf94 Mon Sep 17 00:00:00 2001
From: Nathan Lee <me@nwlee.tech>
Date: Thu, 30 Jul 2026 13:43:03 -0500
Subject: create stub for syscalls

---
 kernel/start.S | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 89 insertions(+), 3 deletions(-)

(limited to 'kernel/start.S')

diff --git a/kernel/start.S b/kernel/start.S
index dce0faa..ecec47b 100644
--- a/kernel/start.S
+++ b/kernel/start.S
@@ -1,10 +1,17 @@
 .section .text._start
 .global _start
 .type _start, @function
+.type _syscall_start, @function
+.type _syscall_exit, @function
+
+.extern user_regs
 
 _start:
-    csrw mstatus, zero
-    csrw mie, zero
+    csrw sstatus, zero
+    csrw sie, zero
+
+    la t0, _syscall_start
+    csrw stvec, t0
     
     la sp, __stack_top
     
@@ -17,4 +24,83 @@ _start:
     bltu t0, t1, 1b
 2:
     call kernel_main
-    j .
+    wfi
+
+_syscall_start:
+    csrw sscratch, x1
+    la x1, user_regs
+
+    sw x2, 4(x1)
+    sw x3, 8(x1)
+    sw x4, 12(x1)
+    sw x5, 16(x1)
+    sw x6, 20(x1)
+    sw x7, 24(x1)
+    sw x8, 28(x1)
+    sw x9, 32(x1)
+    sw x10, 36(x1)
+    sw x11, 40(x1)
+    sw x12, 44(x1)
+    sw x13, 48(x1)
+    sw x14, 52(x1)
+    sw x15, 56(x1)
+    sw x16, 60(x1)
+    sw x17, 64(x1)
+    sw x18, 68(x1)
+    sw x19, 72(x1)
+    sw x20, 76(x1)
+    sw x21, 80(x1)
+    sw x22, 84(x1)
+    sw x23, 88(x1)
+    sw x24, 92(x1)
+    sw x25, 96(x1)
+    sw x26, 100(x1)
+    sw x27, 104(x1)
+    sw x28, 108(x1)
+    sw x29, 112(x1)
+    sw x30, 116(x1)
+    sw x31, 120(x1)
+
+    csrr t0, sscratch
+    sw t0, 0(x1)
+    
+    jal kernel_trap
+
+_syscall_exit:
+    csrw sscratch, x1
+    la x1, user_regs
+
+    lw x2, 4(x1)
+    lw x3, 8(x1)
+    lw x4, 12(x1)
+    lw x5, 16(x1)
+    lw x6, 20(x1)
+    lw x7, 24(x1)
+    lw x8, 28(x1)
+    lw x9, 32(x1)
+    lw x10, 36(x1)
+    lw x11, 40(x1)
+    lw x12, 44(x1)
+    lw x13, 48(x1)
+    lw x14, 52(x1)
+    lw x15, 56(x1)
+    lw x16, 60(x1)
+    lw x17, 64(x1)
+    lw x18, 68(x1)
+    lw x19, 72(x1)
+    lw x20, 76(x1)
+    lw x21, 80(x1)
+    lw x22, 84(x1)
+    lw x23, 88(x1)
+    lw x24, 92(x1)
+    lw x25, 96(x1)
+    lw x26, 100(x1)
+    lw x27, 104(x1)
+    lw x28, 108(x1)
+    lw x29, 112(x1)
+    lw x30, 116(x1)
+    lw x31, 120(x1)
+
+    csrr x1, sscratch
+
+    sret
-- 
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