diff --git a/README.md b/README.md
index d8d45b6..67ff2f3 100644
--- a/README.md
+++ b/README.md
@@ -52,16 +52,27 @@ where:
  - `<kernel>` is the kernel file to boot
  - `<command line>` is the command line to pass to the kernel - don't forget to
    pass `root=` as `mlbinstall` will not calculate the root device. Currently
-   the command line cannot be longer than 40 characters (more just won't fit in
-   the MBR).
+   the command line cannot be longer than 80 bytes (more just won't fit in the
+   MBR).
  - `[-vbr]` will make `mlbinstall` ignore the space for the partition table,
-   giving you more space for the command line. You can safely use it if you're
-   installing to a Volume Boot Record (partition) and not the Master Boot
-   Record, e.g. for using with another bootloader in multi-OS setups.
+   giving you 144 bytes for the command line. You can safely use it if you're
+   installing to a Volume Boot Record (partition), e.g. for using with another
+   bootloader in multi-OS setups.
 
 `mlbinstall` is quite noisy if it detects any problems. Restore your MBR from
 backup, fix the problems, and try again. If it succeeds, it won't say anything.
 
+Error codes
+-----------
+
+If MLB fails to boot your kernel, it will print a one-letter error code.
+Currently, there are two error codes:
+
+    Error What it means
+    ----- ----------------------------------------
+      R   Failed reading data from disk
+      M   Failed moving data to its final location
+
 Contact
 -------
 
diff --git a/mlb.asm b/mlb.asm
index 9fcaeec..dc4c80d 100644
--- a/mlb.asm
+++ b/mlb.asm
@@ -68,13 +68,6 @@ read_kernel_setup:
 	mov	cx, 0x1000
 	call	read_from_hdd
 
-check_version:
-
-	cmp	word [es:0x206], 0x204		; we need protocol version >= 2.04
-	jb	error
-	test	byte [es:0x211], 1
-	jz	error
-
 set_header_fields:
 
 	mov	byte [es:0x210], 0xe1		; set type_of_loader
@@ -149,6 +142,7 @@ read_from_hdd:
 	mov	si, dap
 	mov	dl, 0x80			; first hard disk
 	int	0x13
+	mov	al, 'R'
 	jc	error
 	pop	edx
 	ret
@@ -162,6 +156,7 @@ do_move:
 	mov	ah, 0x87
 	mov	si, gdt
 	int	0x15
+	mov	al, 'M'
 	jc	error
 	pop	es
 	pop	edx
@@ -169,24 +164,12 @@ do_move:
 
 error:
 
-	mov	si, error_msg
-
-msg_loop:
-
-	lodsb
-	and	al, al
-	jz	reboot
 	mov	ah, 0xe
-	mov	bx, 7
+	xor	bh, bh
 	int	0x10
-	jmp	short msg_loop
-
-reboot:
-
-	xor	ax, ax
-	int	0x16
-	int	0x19
-	jmp	0xf000:0xfff0			; BIOS reset code
+.hlt:
+	hlt
+	jmp	error.hlt
 
 ; Global Descriptor Table
 
@@ -223,6 +206,5 @@ dap:
 	dd	0				; low bytes of LBA address
 	dd	0				; high bytes of LBA address
 
-error_msg	db	'err', 0
 current_lba	dd	0xefbeadde		; mlbinstall replaces this with the kernel LBA
 cmd_line	db	0
diff --git a/mlbinstall.c b/mlbinstall.c
index 3e90da4..793d7cd 100644
--- a/mlbinstall.c
+++ b/mlbinstall.c
@@ -19,12 +19,16 @@
  * along with mlbinstall. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#define _BSD_SOURCE
 #define _POSIX_C_SOURCE 200809L
 #define _XOPEN_SOURCE 500
 
 #include <stdint.h>
-#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
 #include <err.h>
+#include <endian.h>
+#include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
@@ -32,12 +36,40 @@
 #include <linux/fiemap.h>
 #include <linux/fs.h>
 #include <sys/ioctl.h>
-#include <unistd.h>
 #include <stdbool.h>
 #include <stdio.h>
 
 #include "mlb_bin.h"
 
+/* Checks if the kernel boot protocol is supported */
+void check_version(const char *kernel)
+{
+	int fd = open(kernel, O_RDONLY);
+	if (fd == -1)
+		err(1, "Failed opening %s", kernel);
+
+	long ps = sysconf(_SC_PAGESIZE);
+	size_t mlength = (512 / ps + 1) * ps;
+	uint8_t *m = mmap(NULL, mlength, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (m == MAP_FAILED)
+		err(1, "Failed mapping %s", kernel);
+
+	uint32_t header = *(uint32_t *)(m + 0x202);
+	uint16_t version = be16toh(*(uint16_t *)(m + 0x206));
+	uint8_t loadflags = m[0x211];
+
+	if (header != 0x53726448)
+		errx(1, "%s is missing a Linux kernel header", kernel);
+	if (version < 0x204)
+		errx(1, "Kernel too old, boot protocol version >= 0x204/\
+kernel version >= 2.6.14 required, but %s is 0x%x", kernel, version);
+	if (!(loadflags & 0x01))
+		errx(1, "Kernel needs to be loaded high");
+
+	munmap(m, mlength);
+	close(fd);
+}
+
 /* Returns the length of cmdline, including the terminating null. */
 uint16_t cmdlen(const char *cmdline, size_t mlblen, size_t mbrlen)
 {
@@ -171,6 +203,8 @@ argument to not reserve space for a partition table and gain an extra\n\
 	const char *kernel = argv[2];
 	const char *cmdline = argv[3];
 
+	check_version(kernel);
+
 	size_t mbr_len = vbr ? 510 : 446;
 	uint16_t cmdline_len = cmdlen(cmdline, mlb_bin_len, mbr_len);
 	uint32_t kernel_lba = lba(kernel);