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.
0x008200 @ 320K fixed variables + program = (md)0x41 to (md)0x300
0x030000 64k sector buffer for current device = (md)0x180 to (md)0x1FF
0x045000 environment variables = (md)0x228 to (md)0x288 approx.
0x060000 end of grub4dos variables + wkspace = (md)0x300
0x07C000 used to load boot code, etc. = (md)0x3E0...
0x090000 may be used by BIOS (avoid) = (md)0x480 to (md)0x4FF
0x0A0000 graphics memory (reserved) = (md)0x500 to (md)0x5FF
0x0C0000 BIOS area (reserved) = (md)0x600 to (md)0x7FF
0x100000 may be used by BIOS (avoid) = (md)0x800 best to avoid up to (md)0x880
0x800000 8M for mem array workspace = (md)0x4000 to (md)0x7FFF
0xA00000 10M-14M for page map = (md)0x5000-0x6FFF (end used by background bitmap buffers - may extend to 0xA000+ if large bitmap?)
Avoid below (md)0x300 and (md)0x4000 to (md)0x7FFF area. Preferably use (md)0x10000 and above (above 32MB).
Use of around (md)0x9000 can cause corruption of bitmap background when a menu is displayed.
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 usable.
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.
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) ...
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 = %@retval%
To display all variables beginning with AB use:
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.
set @extend BASE_ADDR SIZE
The BASE_ADDR is the memory start of extended variable.
SIZE is how many variables it will extended by (MAX is 0xFFFF)
e.g. add 10 variables (so we can have 70 variables in total) at 0x40000.
set @extend 0x40000 10
this needs memory (((10+63)/64) + 10)*512 = 5632.
You must confirm the BASE_ADDR has enough space.
setlocal/endlocal will not work correctly however as any variables over the 60 that are set inside a setlocal section will remain after an endlocal!
So if you are using @extend and setlocal/endlocal, be sure to not use more than 60 variables inside the setlocal/endlocal segments!
set @extend 0x40000 10
set 70 variables
>>> 10 variables will be listed!
The first number is the base address in memory where the extra variable names are stored. The second number is the area where the extra variable values are stored. The last number is the number of extra variables (above 60) that can be stored.
You can get a substring of any variable like this: %variablename:~startpos,endpos%
The startpos shows where the substring begins ( + position from the beginning, - position from the end)
The endpos shows where it ends ( + position from the beginning, - position from the end)
A header file which may help if you understand C code can be found here.
Example - test string length of KEY is exactly 29 characters
if not "%KEY:~29,1%"=="" set KL=1 && echo TOO LONG!
if "%KEY%"=="%KEY:~0,28%" set KL=2 && echo TOO SHORT!
if not "%KL%"=="0" pause --wait=3 INVALID PRODUCT KEY "%KEY%"! && configfile /menu.lst
Filename strings in batch files
In a batch file, %0 is the full path of the batch file itself (if the batch file is in the root then no leading / is output), %~nx0 is the batch files filename + extension, %~n0 is just the filename and %~x0 is just the extension (e.g. .gb4).
n is filename, x is extension, p is the path and d is the drive name.
%0=/xx/fred.g4b (if in root then fred.g4b)
%~pnx0=/xx/fred.g4b (if in root then /fred.g4b)
%~0=parameter without quotes
%~f0=fully path of filename - e.g. if %0=fred.inf then %~f0=(hd1,0)/dir1/dir2/fred.inf if root was set to (hd1,0)/dir1/dir2
If you pass more than one filename to a batch file, %1 will be the second parameter, %2 the third, etc. %* is all parameters.
Batch file to get all parameters as arg1...argn and argc as number of parameters:
if "%1"=="" goto :fin
set /a argc=%argc%+1 > nul
Note: BEWARE! in batch files beware of %0 %1 %2 accidents! e.g.
will not display 32 (it displays %f because %1, %2 %3 special variables are substituted first, so grub4dos translates this to %f+%2) - instead use this...
%% in a batch file is reduced to %. Since %%2 does not mean anything to grub4dos on the first substitution pass, we get the expected answer.
If you have a filename or full or partial filespec, you can get bits of it like this:
If we have a number of variables FRED1, FRED2, FRED3, etc, and we want to print out the value of each, we can use a counter like this
set /a N=%N%+1 > nul
call echo %^FRED%N%%%
if %N%<=2 goto :LOOP
Note that using ^ anywhere inside a variable prevents it from being translated into a value. So %^FRED%N%%% gets translated by 'call' to %FRED1% and then echo displays the FRED1's value.
If not using a batch file use call echo %^FRED%N%% instead (in a batch file, %%=%).
Internal named variables
grub4dos has a few predefined variables - these can be used in a grub4dos command - e.g. echo %@date% or set d=%@date%
These are case sensitive:
@date - e.g. 2012-04-08
@time - e.g. 15:29:11
@random - 0 to 32767 - e.g. set/a num1to10=%@random%/3277+1 or set /a num1to10=%@random:~-1,1% + 1
@root - e.g. (hd0,0)
@path - e.g. (bd)/BOOT/GRUB/
@retval - e.g. 1 - NOTE: This is a 32bit value ranging from -2GB to +2GB
?_BOOT - disk&partition that contained the file \grldr that was used to boot to grub4dos, e.g. (hd0,0) - note: this is NOT the same as (bd) which is set to whatever partition contained the menu.lst file.
?_UUID - set after uuid command - e.g. uuid (bd)
? - result of last command - e.g. cat --locate=string --number=1 /myfile ;& set offset=%?% ;& echo string found at %offset% prints 0x1de or 0x0 if not found, @retval is 0 if not found or <count> if found
You can set the default file extensions of grub4dos executables using command --set-ext= (this is similar to typing just fred in the Windows command shell and Windows would run fred.exe) - for example, we can set the default extensions like this:
If you then have a command like fred or /fred in a menu or batch file, grub4dos will try “fred” first and then fred.g4b and then fred.g4x.
You can call an internal function like sprintf within a grub4dos menu or a grub4dos batch file. For example:
call Fn.0 0 "stringa=%s stringb=%s" %a% %b%
this prints on the display
Here is a list of the Fn calls. I don't know all the details for all the parameters, but each parameter should be separated by a space.
Note that Fn. is case sensitive and must have a capital F and a lowercase n.
Some equivalent C library functions for these can be searched for here.
0 grub_sprintf syntax: call Fn.0 <memory location> <printf string> - if <memory location> = 0 then is output to screen, if <memory location>=0x60000 then the string is output to memory at (md)0x300. @retval returns the length of the string - e.g. call Fn.0 0x60000 "%pci%\r\n" = call func sprintf("%pci%") - another example: call Fn.0 0 "id="%s","%1"" %p_hwid% - This function is also useful for stripping quote marks from variables (but ensure there is only one pair or things can go wrong! Also note that Fn.0 0 fred will output 'fred' but Fn.0 0 1234 will not work as it is treated as a number - use Fn.0 0 "1234" instead.) call Fn.0 0 0xFFFF5 | set biosdate= will return the date of the BIOS on most systems - e.g. 06/23/99
2 putchar - e.g. call Fn.2 49 prints '1' (if in the text mode console, call Fn.2 7 will 'beep' the internal speaker if your BIOS and hardware supports it - i.e. if it is a PC and has a 'beeper-speaker' fitted!)
3 get_cmdline_obsolete 4 getxy The return value is ((X << 8) | Y). 5 gotoxy - e.g. call Fn.5 0 11 set cursor pos to beginning of line 11 6 cls
7 wee_skip_to was obsolete setcursor 8 nul_terminate 9 safe_parse_maxint_with_suffix 10 substring 11 grub_strstr - see here - find string in string, returns @retval as position of string start, e.g. call Fn.11 0x6000 "$" || exit or call Fn.11 "%filefind%" "0" && echo found 0 in %filefind% 12 grub_strlen - e.g. call Fn.12 "freddy" ;; echo %@retval% - prints 6 13 grub_strtok - seeherefor definition e.g. echo 123,456,789 > (md)0x200+1 ;; call Fn.13 0x40000 "," ;; echo First string at %@retval% ;; call Fn.13 0 "," ;; echo Next string at %@retval% ;; cat --skip=262152 (md)0+0x201 (note: memory is changed by each call a 0 is inserted at the delimiting character position -> 123<0>456<0>789
14 grub_strncat - Appends the first num characters of source to destination, plus a terminating null-character 15 grub_strcmp compare two strings - e.g. call Fn.15 XXX XXX ;; echo %@retval% returns 0 (or -1 if no match) 16 grub_strcpy - copies string into destination
18 reserved 19 getkey - gets a kbd key from BIOS kbd buffer 20 checkkey- get kbd key without emptying buffer - e.g. call Fn.20 ;; set /A key=%@retval%&0xff > nul ;; echo %key% prints 0x31 if number 1 key pressed (top 8 bits is scancode, bottom 8 are ASCII code, Function keys are 'special') 21 22 grub_memcmp
24 grub_memset call Fn.24 0x83562 0xaa 0x100 - fill address 0x83562 in memory with byte 0xaa for size of 256 bytes.
25 26 grub_open - e.g. directly call grub_open file function it will put filesize at memory 0x8320 - this works well for non-compressed files too. call Fn.26 /myfile.gz ;; set /a filesize=*0x8320 27 grub_read
41 parse_string 42 hexdump - e.g. call Fn.42 0x8000 0 3 - list 3 hex bytes at 0x8000
53 realmode_run - executes a BIOS interrupt in real mode - e.g. see date.g4b batch file for example.
54 reserved for wee
55 reserved for wee
56 reserved for wee
57 reserved for wee
58 reserved for wee
59 reserved for wee
60 reserved for wee 61 dir - e.g. Fn.61 /dir/
67 unicode_to_utf8 e.g. call Fn.67 *0x82d0 0x60000 3 cat (md)0x300+1,1 | set nt_ver=NT
69 rawwrite 70 setcursor x (returns previous cursor state as %@retval%) e.g. call Fn.70 0 to disable cursor and splashimage, call Fn.70 3 && clear will keep the splashscreen visible in console mode (but scrolling doesn't work when the cursor gets to the bottom of the screen so must clear the screen when get to bottom!) - 1=show cursor and disable splashimage, 2=normal splashimage mode,
72 grub_isspace 73 grub_sleep e.g. call Fn.73 3 to sleep for 3 secs
77 rectangle (x y w h border_width) - e.g. call Fn.77 180 180 100 100 2 - values in pixels not lines. Can be called multiple times for multiple rectangles on screen.
Internal Variable Locations held in memory
When grub4dos loads into memory, it will store certain internal, local values at defined places within memory. Sometimes it is useful to read or write to these locations.
For instance, the grub4dos version number is held at location 0x8278 in memory. So you can check what version the user is running by adding these lines to your grub4dos menu.lst file:
checkrange 20120201:-1 read 0x8278 || pause --wait=3 Please use grub4dos-0.4.5c-2012-02-01 or later! && exit 1
This checks the value of 0x8278 to ensure it is between 20120201 (the date that the grub4dos version was made) and -1 (which is the largest possiblenumber). If this check fails then the message is displayed with a 3 second countdown and then the menu is exited.
Some variable locations + examples are given in the lists below: Use latest version available or these may not work!
0000:82A04 (DWORD)saved_drive (current root drive) set /a cur_drv=*0x82A0&0x7F
0000:82A44 (DWORD)no_decompression (no auto gunzip) 1=no decomp
0000:82A88 (QWORD)part_start (start sector of last partition)
0000:82B08 (QWORD)part_length (total sectors of last partition)
0000:82C0 8 (QWORD) saved_mem_higher (max contiguous mem in KB starting at 4G
0000:8304 4 (DWORD) ptr to current graphics mode - set /a n=23<<2+0x8304&0xffffffff ;; set /A n=%n%&0xffffffff ;; echo Current graphics mode is %n% (0=text mode)
0000:8308 1 (BYTE) symbol at left of highlighted menu entry (default=0x10 ►) - use 0 if not wanted or 0x1a for →
grub4dos 0.4.6 only
0000:8350 1 (BYTE) number of USB drive loaded by usb driver after usb --init command - e.g. 01 - read 0x8350 & 0xff
0000:8351 8 (BYTES) USB drive numbers loaded by internal g4d USB driver in order e.g. 0x808182
0000:8359 4 (DWORD) Base address of USB driver data area
set /a MEMSIZE=*0x8298 & 0xffffffff >> 10+1
set /a MEMSIZE1=*0x82c0 >> 10+1
set /a TMEM=%MEMSIZE% + %MEMSIZE1%
set /a TMEMG=%TMEM% / 1024
echo Total Memory = %TMEMG%MB
0x8217 current configfile path and name e.g. call Fn.0 0 0x8217 0x826C BSS start address (4 bytes) 0x8274 autonumber boot entries - e.g. use a hyphen after the number - write 0x8274 0x2d01 (0x2001=default using space, 0 = do not number menu entries) 0x8276 set /a CURDEF=*0x8276 & 0ff gets current menu item number as a variable CURDEF (0x8277 is not used) 0x8278 check version of grub4dos - e.g. checkrange 20120201:-1 read 0x8278 || pause --wait=3 Please use grub4dos-0.4.5c-2012-02-01 or later! && exit 1
0x8280 Disk type - checkrange 0x80 read 0x8280 && pause --wait=3 I am Hard Disk 0
0x8290 Length of file in bytes -e.g cat --length=0 /myfile.iso ;; set /a LEN=*0x8290 ;; echo Length of file is %LEN% bytes (use if files could be over 4GB) 0x8298 maximum free memory in KB starting at 1M and below 4G (memory may NOT be contiguous) # calculate sizes in MB of iso and available memory - set /a MEMSIZE=*0x8298&0xFFFFFFFF>>10 ;; cat --length=0 /myfile.iso ;; set FSize=*0x8290>>20 ;; if %FSize%>=%MEMSIZE% echo Need More memory! 0x82BC CPU type - iftitle [checkrange 0,1 read 0x82Bc] 32bit system 0x82D0 rd_base 0x82D8 rd_size 0x8320 to get the expanded length of a compresses .gz file - use: cat --length=1 /myfile.gz ;& set /a filesize=*0x8320
# Or directly call grub_open file function it will put filesize at memory 0x8320 - this works well for non-compressed files too. call Fn.26 /myfile.gz ;& set /a filesize=*0x8320 0x8328 filepos ptr
set /a IP1 = *0x8284 & 0xFF
set /a IP2 = *0x8285 & 0xFF
set /a IP3 = *0x8286 & 0xFF
set /a IP4 = *0x8287 & 0xFF
set YIP = %IP1%.%IP2%.%IP3%.%IP4%
0x307FF4 can be set to change the address of where grub4dos environment variables are stored (default value 0x45000) - e.g. write 0x307ff4 0x400000 && set * && set ?_BOOT=%?_BOOT%
To patch grldr so that no diagnostic messages are displayed by it whilst it loads, patch byte at 0x2073 (takes affect after reboot)
# Silent startup of grub4dos - no diagnostic/info messages (only works for 0.4.6a) - permanently changes /grldr files
For silent stage 1.5 booting, use new grubinst.exe and set silent switch.
New commands August 2014
write --bytes=1 0x8277 12 - writes a single byte to address 0x8277 (0x8277 is unused in grub4dos)
crc32 ()/fred.iso - calculates crc32 of file
Other internal system variables
Note: these are undocumented and they could change at any time! Use them only if you have to:
read VAR.42 ;; set /A color_32=%@retval% ;; # e.g. 0x0B
read VAR.43 ;; set color_fg=%@retval% ;; # e.g. 5636095
VAR.13 - filesystem type ????
VAR.28 - graphics cursor
VAR.29 - menu border character?
VAR.42 - current color BG-FG as byte
VAR.43 - low 32-bits foreground
VAR.44 - low 32-bits background
VAR.45 - cmd line string
VAR.46 - splashimage loaded if not 0
The 0x8304 below could change - use VAR.xx instead!
Getting the current standard (console) text and background colours from internal system variables: 64-bit background colour - calc 44<<2 + *0x8304 ;; read %@retval% ;; set color_bg=%@retval%
64-bit text colour - calc 43<<2 + *0x8304 ;; read %@retval% ;; set color_fg=%@retval%
8-bit color if in textmode (first 4 bits are background, next 4 bits are text colour) - calc 42<<2 + *0x8304 ;; read %@retval% ;; set color_fb=%@retval%
See PrintMsg.zip for a batch file which prints a message in any colour but preserves the background colour. This works in both text mode and graphics mode and you don't need to know what the current text and background colours are.
echo $[0x04] Hello - prints text in red on black background
echo $ Hello - prints text in red on current background (if in graphicsmode)
Other useful stuff
use latest version of grub4dos for these features!
debug 0 Off - No messages debug 1 Normal - informational messages ouput debug 2 On - Extra info debug 3 May display more messages (?) debug -1 Do not show timer countdown in pause --wait=x command debug = 0x7FFFFFFF 'internal' debug messages displayed
debug <batchfile> <args> - Single-Step through a batch file - e.g. debug /grub/listfiles.g4b (hd0,1)/ [ENTER] - runs next line Q - quit execution C - return to the shell (ESC will continue execution where it left off) S - Skip line (does not execute the current line) E - End stepping - will run normally until the end of the batch file or until it hits a break point B - Set Breakpoint - press B then enter a line number. Then press E and it will run and break at the line that was set. N - Run until next function (e.g. next call command)
debug msg=n (0.4.6 only)
0 ==> will disable all messages, even debug 2 level ones 1 ==> show stdmsg to stdout, warning and error messages to stderr 2 ==> show stdmsg and warning to stdout, error messages to stderr 3 ==> show all message to stdout
If using calc in grub4dos, some operators may not work with spaces and some may work - e.g. calc 44|2 works, but calc 44 | 2 does not. operators | % >> and << cannot have spaces, but * / - + ^ & work with spaces or without.
To get a key press and act on it:
pause --test-key --wait=5
:: read BIOS key code location
set /A key=*0x4CB00
:: key has 0x01 if not key else key scan code
:: scancode for p is 0x1970 P is 0x1950
:: if p is pressed wait indefinitely for another key press
if %key%==0x1970 && pause --test-key && set /A key=*0x4CB00
:: Now jump to a routine to deal with key or do nothing if there is no label
goto :%key% || echo -n
:: do stuff here if no key pressed
:: do stuff here for F5 keypress
# get the UUID of a volume containing the menu.lst file we used to run grub4dos (note: do NOT use && to join these lines!)
uuid (bd) > nul
or you can use just one line:
uuid () && call set UUID=%^?%
Note that %^?% is used because %?% would be treated as a literal string and UUID would be set to %?% if ^? was not used! ^ anywhere inside a variable means do not translate to a variable - e.g. set a=%bb^bbb% returns a=%bbbbb%
Remove quotes from a string
call Fn.0 0 %id% | set id=
The ! operator
! can only be used after a && or || operator to mean 'else'
if %e%>=10 if %e%<=90 && echo fred || echo doris Nothing is displayed as result of echo fred was true
if %e%>=10 if %e%<=90 && echo fred ! echo doris 'doris' is displayed as result of if 9>-10 if 9<=90 was false
grub4dos will not allow writes to a compressed file. If grub4dos tries to read bytes from a compressed file it will try to decompress the file first.
If you are using an un-initialised memory area, you may find that the write to memory does not work on some systems or on some occasions.
write (md)0x3000+1 fred
may not write the string 'fred' to the memory area - this is usually because grub4dos thinks that the memory area being used holds a compressed file (which may just be random bytes of memory!). To overcome this you can either temporarily turn off compressed file (gzip and lzma) support using 0x82a4 or ensure the start of the memory area does not contain random bytes, as follows:
# turn off compressed file support
write 0x82a4 1
write (md)0x3000+1 a string
write 0x82a4 0
# initialise memory first
echo ffffffffffffffffffffffffffff > (md)0x3000+1
write (md)0x3000+1 a string
equally cat --hex (md)0x3000+1 may not work if grub4dos thinks that the file is a compressed file and if it cannot decompress it.
cat --hex /fred.gz --- will list the decompressed contents (if fred.gz is a valid compressed file)
cat --hex (md)0x3000+1 --- will try to list memory, but if it thinks the contents contain a compressed file and grub4dos cannot decompress it then it won't list anything! So for listing bytes from memory, always use:
write 0x82a4 1
cat --hex (md)0x3000+1 - will always work!
write 0x82a4 0 - re-enable compressed file support
An example of using @retval
debug -1 ;; pause --wait=10 Press a letter ;; set /A a=%@retval%+0 > nul ;; if %a%>=0x20 echo -e \%a:~1% was pressed (%a%)!
Press a letter (user presses d on keyboard)
d was pressed (0x64)!
(0x1 is returned by pause --wait=10 if no key is pressed within 10 seconds.)
cat --locate=fred (md)0x3000+1 will return @retval = count of number of instances found.
Note that nearly all commands will change the value of @retval, so copy it to a variable if you need to keep it, e.g.
cat --locate=fred (md)0x3000+1
echo fred occurs %count% times
Mapping a new hard disk
(hd) is the next available (free) harddisk number not yet recognised by the BIOS. (hd-1) is the last BIOS harddisk.
e.g. If you have 2 hard disks in the system (hd0 and hd1), then ...
map /fred.img (hd) - maps fred.img to the next available hard drive number (hd2) and increases BIOS harddrive count by 1
ls (hd-1,0)/ - lists files on the last hard drive (hd1,0)
map /doris.img (hd) - maps doris.img to the next available hard drive number (hd3) and increases BIOS harddrive count by 1
ls (hd-2,0)/ - lists files on the first hard drive (hd0,0)
map --hook - make BIOS mappings take effect
root (hd-1,0) - makes the last available hard disk first partition (in doris,img) the root
ls (hd-1,0)/ - lists files in doris.img partition 0
ls (hd3,0)/ - lists files in doris.img partition 0
Equivalent of dd if=/dev/zero to fill a file with zeros or spaces
If you want to fill a file with the same character or 00 byte, you can fill an area of memory using the memset function first and then use it as if:
# fill (rd) with 0's , 0x82d0 is rd-base mem address, Fn24 is memset - fill memory <addr> <string> <size> - 200 sectors = 102400 bytes
If you are not sure how large the target file is, then make the (md) area larger than the file. To fill with spaces, substitute 0x00 with 0x20.
To fill an area of memory (e.g. 10*512=5120 bytes) with 0's the following trick can be used (see below for details of why this works):
if 1=2 echo never > (md)0xa000+10
Sometimes echo will not redirect correctly to a file or memory - e.g. when it contains a $[xxxx] string...
echo $fred > (md)0x300+1
will only put 'fred' in memory and the $ will not be redirected.
To circumvent this do not to use echo - e.g.
pause --wait=0 $fred > (md)x0300+1
call Fn.0 0 $fred > (md)x0300+1
write (md)0x300+1 $fred
write (md)0x200+1 $fred\0
cat (md)0x200+1 > (md)0x300+1
Redirection using > and parsing gotchas!
When grub4dos finds a > symbol, it will open a file handle to the specified file, clear it's contents if it already exists and then redirect all standard output into it. So
echo fred > (md)0xa000+4
will clear 4 sectors in memory (2048 bytes) and then place the text fred in the first 4 bytes.
Also note that due to the way a command line is parsed, the following lines will always fill 2048 bytes of memory at (md)0xa000 with 0's even though fred will never be echo'd
if %A%=2 echo fred > (md)0xa000+4
this can be fixed by using a ;; or && command to force the parser to execute each section separately:
if %A%=2 && echo fred > (md)0xa000+4
Prompt with timeout in colour (grub4dos batch file)
Here is an example of how to change the text colour temporarily (until echo -n is used). Instead of set /p%TIME% ask= you could use password fred and then both the Password: prompt and the ***'s printed would be in the pre-set colour.
# Get a string from user after prompting in coloured text
# Syntax: ColPrompt <colour> <"prompt"> [timeout in secs]