Grub4dos Internal variables, memory areas, advanced features and function calls

counter customisable

The following is a list of some grub4dos internal variables and functions from a 2013 or later version of a chenall build of grub4dos (as used in RMPrepUSB).

Memory areas used by grub4dos

The physical memory address range from 1M to 32M is reserved for grub4dos internal code and data. Grub4dos may fail to run on systems with less than 32MB of RAM. Ideally users should not use this range (32MB = 0x200,0000 = below (md)0x10000).
Some use of (md)0x300 is relatively safe if low memory is required.
Some areas below 1MB are also used by grub4dos.

Grub4dos executes grub4dos executables and kernel payloads at 32MB in memory, therefore it is best to use memory areas well above 32MB for any memory workspace if you intend to call grub4dos executables after loading kernel files, etc.

Note: In grub4dos, batch files and programs run in the same piece of memory as the 'kernel' command. Therefore you cannot use the kernel command directly from a batch file or grub4dos program.
To workaround this, either allocate memory to move the buffer by using call Fn.50 0x200000 as the very first line in menu.lst OR use configfile, e.g.
echo kernel /xxx/vmlinuz aaa bbb ccc > (md)0x220+1
echo initrd initrd.gz >> (md)0x220+1
configfile (md)0x220+1

(md) is a memory 'device'. (md)x+y where x is the start sector in memory and y is the length of memory in sectors (e.g. (md)4+2 = start at 2KB in memory for a length of 1KB. Note that the BIOS uses low memory!

Grub4dos use of memory - (md) uses 512-byte 'sectors':
0x0008200 320K fixed variables + grldr program = (md)0x41 to (md)0x300
0x0030000 64k sector buffer for current device = (md)0x180 to (md)0x1FF
0x0045000 environment variables = (md)0x228 to (md)0x288 approx.
0x0060000 end of grub4dos variables + wkspace = (md)0x300
0x007C000 used to load boot code, etc. = (md)0x3E0...

0x0090000 may be used by BIOS (avoid) = (md)0x480 to (md)0x4FF
0x00A0000 graphics memory (reserved) = (md)0x500 to (md)0x5FF
0x00C0000 BIOS area (reserved) = (md)0x600 to (md)0x7FF
0x0100000 may be used by BIOS (avoid) = (md)0x800 best to avoid up to (md)0x880

0x0800000 8M for mem array workspace = (md)0x4000 to (md)0x7FFF
0x0A00000 10M-14M for page map = (md)0x5000-0x6FFF
0x1000000 16M-19M used to store background bmp = (md)0x8000-0xD460 (end of buffer depends on size of bitmap, 1920x1440=0xD460, 800x600=0x8EA6, 1024x768=0x9800)

Avoid below (md)0x300 and (md)0x4000 to (md)0x7FFF area. Preferably use (md)0x10000 and above (above 32MB).

Use of around (md)0x8000 to 0x9800 can cause corruption of bitmap background when a menu is displayed in 1024x768 mode (even more used in higher res modes!).
However wenv/insmod seems to load files above 32MB - therefore use above 50MB = (md)0x19000 to be safe!

Many BIOSes use the last part of segment 9000:0 for a scratchpad. All IBM-compatible BIOSes use A000:0 - F000:FFFF.

Black areas are normally unused by grub4dos.

50 40 0.039 A000 000A00:0
60 48 0.0468 C000 000C00:0
80 64 0.0625 10000 001000:0
220 272 0.265 44000 004400:0
300 384 0.375 60000 006000:0
400 512 0.5 80000 008000:0
500 640 0.625 A0000 00A000:0
600 768 0.75 C0000 00C000:0
700 896 0.875 E0000 00E000:0
800 1024 1 100000 010000:0
880 1088 1.0625 110000 011000:0
900 1152 1.125 120000 012000:0
a00 1280 1.25 140000 014000:0
b00 1408 1.375 160000 016000:0
c00 1536 1.5 180000 018000:0
d00 1664 1.625 1A0000 01A000:0
e00 1792 1.75 1C0000 01C000:0
f00 1920 1.875 1E0000 01E000:0
1000 2048 2 200000 020000:0
2000 4096 4 400000 040000:0
3000 6144 6 600000 060000:0
3100 6272 6.125 620000 062000:0
3200 6400 6.25 640000 064000:0
4000 8192 8 800000 080000:0
5000 10240 10 A00000 0A0000:0
6000 12288 12 C00000 0C0000:0
7000 14336 14 E00000 0E0000:0
8000 16384 16 1000000 100000:0
9000 18432 18 1200000 120000:0
a000 20480 20 1400000 140000:0

(md)0x200 - 0x220 this seems to be used by chenall, et al. and so is probably safe!
(md)0x300 - 0x4FF 384-600KB approx. (0x3E0 may be used for chainloading by grub4dos)
(md)0x1000 - 0x3FFF 2-8MB
(md)0xD800 - 0xFFFFF (can start from 0x9800 as long as not using higher Res than 1024x768)
(md)0x19000 + (50MB+)

As most systems will have over 50MB of memory, use memory after (md)0x19000 to be ultra safe!

Using grub4dos variables

Grub4dos variables recognises a maximum name length 8 characters and each variable holds a maximum size of 511 characters or 512 bytes. The variable name is case sensitive, e.g.

set a=1
set A=2


e.g. do not exceed 8 characters for variable names or it could cause unexpected problems (FRED12345 in this example is shortened by grub4dos to FRED1234) ...

set FRED1234=Hi
set FRED12345=Lo
echo %FRED1234%
echo %FRED12345%


Note: Variables and their values are stored in memory and may share the same workspace area as modules loaded using the 'insmod' command. You may get crashes if you define too many variables and have lots of large modules loaded into memory (use delmod to remove the modules from memory after use).

#clear all variables except fred
set * && set fred=%fred%

You can define leading spaces in a variable and have characters like && or ;; by enclosing the argument in double-quotes (must start as "<variable>= with no spaces
set "a= fred && doris"

Be careful using && or || as the environment is not changed until the whole line has been executed - e.g. the following line will not print HELLO the first time it is run because A is not set until after the whole line has been executed:

set A=HELLO && echo %A%

Get the number of characters in any variable - this example prints 5
# set command will return the length
set a=freda
# do stuff here...
# get length of a
set a=%a%
echo Number of characters in variable a = [email protected]%

To display all variables beginning with AB use:
set AB
You can use IF EXIST to test for the existence of a variable

if exist FRED echo Variable FRED exists!

Increasing the variable space

Grub4dos has space for 60 variables, each one can have a value size of 512 bytes.

# check if a variable has been defined
if exist FRED1234 echo FRED1234 is set to %FRED1234% || echo Variable FRED1234 has not been defined!

In the latest versions of grub4dos (15 May 2013 and later) you can extend the variable space using an undocumented @extend command:
You can set up to max 65,536 variables using @extend.