MBR bootloader: fix 15/16 MB kernel limit

[stapelberg]

Platform

I’m using: gokrazy/apu2c4, gokrazy/x86-64

Observed behavior

The bootloader that gokrazy is currently shipping has a limit (16 MB architecturally, 15 MB in practice) for the kernel size:

https://michael.stapelberg.ch/posts/2024-02-11-minimal-linux-bootloader-debugging-story/

Options

1. Investigate using syslinux
2. Attempt to fix the issue in Minimal Linux Bootloader (probably won’t work on this)

[stapelberg]

From https://lobste.rs/s/kaj3c2/minimal_linux_bootloader_debugging#c_ybraf4, someone (with the nick name abbeyj, thank you!) suggested that the fix is to add a adc byte [gdt.dest+5], 0 instruction where the number of bytes is written to the GDT, and indeed this diff seems to work:

diff --git i/mbr/bootloader.asm w/mbr/bootloader.asm
index 58aa030..eb3dfd5 100644
--- i/mbr/bootloader.asm
+++ w/mbr/bootloader.asm
@@ -119,6 +119,7 @@ read_protected_mode_kernel:
 	sub	edx, 0xfe00			; update the number of bytes to load
 	add	word [gdt.dest], 0xfe00
 	adc	byte [gdt.dest+2], 0
+	adc	byte [gdt.dest+5], 0
 	jmp	short read_protected_mode_kernel.loop
 
 read_protected_mode_kernel_2:
@@ -186,18 +187,14 @@ do_move:
 	ret
 
 error:
-
 	mov	si, error_msg
 
 msg_loop:
 
-	lodsb
-	and	al, al
 	jz	reboot
 	mov	ah, 0xe
 	mov	bx, 7
 	int	0x10
-	jmp	short msg_loop
 
 reboot:
 

I’ll properly remove the error function (instead making the code just reboot) to make space for the extra instruction, then add padding so that we end up at 440 bytes again.