fix bios call, return register values to caller

boot1/asm/bios_call.s

@@ -12,7 +12,7 @@ section .text
 [bits 32]
 
 ; FIXME: use a separate stack for real mode
-bios_call:
+_bios_call:
   .PARAMS_START             equ 36
   .PARAM_BIOS_INTR_NUM      equ .PARAMS_START
   .PARAM_BIOS_INTR_ARGS_PTR equ .PARAMS_START + 4
@@ -83,13 +83,30 @@ bios_call:
   call [bp + 2]
   pop bp
 
-  ; TODO: return resulting registers to the caller
+  ; Copy the resulting register values back into the `BiosIntr` struct provided by the caller.
+  pushfd
+  push es
+  push ds
+  push edi
+  push esi
+  push edx
+  push ecx
+  push ebx
+  push eax
+  mov cx, 32
+  mov si, sp
+  mov di, [bp + .PARAM_BIOS_INTR_ARGS_PTR]
+  rep movsb
+  add sp, 32
 
   ; Reset the segment registers that were changed to the caller-provided values.
   xor ax, ax
   mov ds, ax
   mov es, ax
 
+  ; Pop the `int` function off the stack.
+  add sp, 4
+
   ; Reload the GDT in case the BIOS call changed it.
   mov bx, sp
   lgdt [bx]
@@ -116,7 +133,7 @@ bios_call:
   popad
   ret
 
-global bios_call
+global _bios_call
 
 
 %macro int_fn 1

boot1/src/main.rs

@@ -25,7 +25,7 @@ struct BiosIntr {
 }
 
 unsafe extern "C" {
-  unsafe fn bios_call(intr: u32, args: *mut BiosIntr);
+  unsafe fn _bios_call(intr: u32, args: *mut BiosIntr);
 }
 
 #[panic_handler]
@@ -54,7 +54,7 @@ pub extern "C" fn _start() -> ! {
   };
 
   unsafe {
-    bios_call(0x15, &raw mut args);
+    _bios_call(0x15, &raw mut args);
   }
 
   for (i, b) in STR.iter().copied().enumerate() {