From: Keith Owens <>
Date: Fri, 11 Jan 2002 16:50:42 +1100

Time for another version of my VIDEO_CHAR debugging patch.

If a kernel hangs early in the boot process (before the console has been initialized) then printk is no use because you never see the output. There is a technique for using the video display to indicate boot progress so you can localize the problem. Reporting "my kernel hangs during boot at line nnn in routine xyz" is a lot better than "my kernel hangs during boot".

The idea is to write characters direct to the video screen during booting using a macro called VIDEO_CHAR. This macro takes a character position and a single character value to be displayed. Use different positions on the screen for different levels of code and use different characters in one position to indicate which stage that level is up to. For example, with the patch below, the string EAC at hang indicates parse_options(), checksetup().

Other people have patches for early printk output and those patches might be better for your needs. The beauty of VIDEO_CHAR is that it does not require any registers on ix86, so it can be used in Assembler without save/restore hassles.

The patch below is generic, except for the definition of VIDEO_CHAR which is ix86 specific. If this patch ever becomes part of the main kernel then VIDEO_CHAR needs to be moved to an arch specific header. If any arch other than ix86 uses this technique, please mail <> with your definition of VIDEO_CHAR.

You can plant VIDEO_CHAR calls anywhere you like, up to the call to mem_init(). After mem_init has done its work and memory has been remapped, VIDEO_CHAR cannot write to video memory, it will oops. However by then the console has been initialized so you can use printk.

Demonstration patch against 2.4.18-pre3. This only uses screen positions 0, 1, 2. If you want to drill down into lower level routines, just use screen positions 3 onwards. To activate the debugging, add

to the start of init/main.c. If the machine is hanging then a zero delay is fine, if it is rebooting then you need a delay to note the characters in the top left hand corner of the screen before it reboots.

A value around your clock speed gives a delay of approx. 1 second.

Index: 18-pre3.1/init/main.c
--- 18-pre3.1/init/main.c Sat, 01 Dec 2001 11:29:21 +1100 kaos (linux-2.4/k/11_main.c 644)
+++ 18-pre3.1(w)/init/main.c Fri, 11 Jan 2002 16:46:41 +1100 kaos (linux-2.4/k/11_main.c 644)
@@ -79,6 +79,16 @@ extern int irda_device_init(void);
 #error Sorry, your GCC is too old. It builds incorrect kernels.
+/* ix86 specific */
+#define VIDEO_CHAR(c, v) { int i; *((volatile char *)(0xb8000 + 2*(c))) = (v); for (i = 0; i < VIDEO_CHAR_DELAY_COUNT; ++i) ; }
+#define VIDEO_CHAR(c, v)
 extern char _stext, _etext;
 extern char *linux_banner;
@@ -425,12 +435,14 @@ static void __init parse_options(char *l
        char *next,*quote;
        int args, envs;
+       VIDEO_CHAR(1, 'A');
        if (!*line)
        args = 0;
        envs = 1;       /* TERM is set to 'linux' by default */
        next = line;
        while ((line = next) != NULL) {
+               VIDEO_CHAR(2, 'A');
                 quote = strchr(line,'"');
                 next = strchr(line, ' ');
                 while (next != NULL && quote != NULL && quote < next) {
@@ -443,9 +455,11 @@ static void __init parse_options(char *l
                                 next = strchr(next+1, ' ');
+               VIDEO_CHAR(2, 'B');
                 if (next != NULL)
                         *next++ = 0;
                if (!strncmp(line,"init=",5)) {
+                       VIDEO_CHAR(3, 'A');
                        line += 5;
                        execute_command = line;
                        /* In case LILO is going to boot us with default command line,
@@ -456,8 +470,10 @@ static void __init parse_options(char *l
                        args = 0;
+               VIDEO_CHAR(2, 'C');
                if (checksetup(line))
+               VIDEO_CHAR(2, 'D');
                 * Then check if it's an environment variable or
@@ -473,9 +489,12 @@ static void __init parse_options(char *l
                        if (*line)
                                argv_init[++args] = line;
+               VIDEO_CHAR(2, 'E');
+       VIDEO_CHAR(1, 'B');
        argv_init[args+1] = NULL;
        envp_init[envs+1] = NULL;
+       VIDEO_CHAR(1, 'C');
@@ -548,16 +567,27 @@ asmlinkage void __init start_kernel(void
  * Interrupts are still disabled. Do necessary setups, then
  * enable them
+       VIDEO_CHAR(0, 'A');
+       VIDEO_CHAR(0, 'B');
+       VIDEO_CHAR(0, 'C');
+       VIDEO_CHAR(0, 'D');
        printk("Kernel command line: %s\n", saved_command_line);
+       VIDEO_CHAR(0, 'E');
+       VIDEO_CHAR(0, 'F');
+       VIDEO_CHAR(0, 'G');
+       VIDEO_CHAR(0, 'H');
+       VIDEO_CHAR(0, 'I');
+       VIDEO_CHAR(0, 'J');
+       VIDEO_CHAR(0, 'K');
         * HACK ALERT! This is early. We're enabling the console before
@@ -565,8 +595,10 @@ asmlinkage void __init start_kernel(void
         * this. But we do want output early, in case something goes wrong.
+       VIDEO_CHAR(0, 'L');
+       VIDEO_CHAR(0, 'M');
        if (prof_shift) {
                unsigned int size;
@@ -577,10 +609,14 @@ asmlinkage void __init start_kernel(void
                size = prof_len * sizeof(unsigned int) + PAGE_SIZE-1;
                prof_buffer = (unsigned int *) alloc_bootmem(size);
+       VIDEO_CHAR(0, 'N');
+       VIDEO_CHAR(0, 'O');
+       VIDEO_CHAR(0, 'P');
+       VIDEO_CHAR(0, 'Q');
        if (initrd_start && !initrd_below_start_ok &&
                        initrd_start < min_low_pfn << PAGE_SHIFT) {
@@ -588,6 +624,7 @@ asmlinkage void __init start_kernel(void
                    "disabling it.\n",initrd_start,min_low_pfn << PAGE_SHIFT);
                initrd_start = 0;
+       VIDEO_CHAR(0, 'R');


KernelNewbies: Documents/DebugWithoutPrintk (last edited 2017-12-30 01:30:01 by localhost)