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



Grub4dos use of memory:
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.

(MD) KB MB MEMORY SEGMENT ADDRESS
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

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
set

a=1
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%
set

Lo
Lo
FRED1234=Lo

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"


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.
Syntax:

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!

e.g.
set @extend 0x40000 10 
set *
setlocal
set 70 variables
endlocal
set
>>> 10 variables will be listed!

The command:

set @extend

will display:

BASE_ADDR:40000,40200,VARS:10

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.

Strings

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)

e.g

set a=0123456789abcdefghijklmnopqrstuvwxyz
echo %a:~0,3%    displays    012
echo %a:~2,3%    displays    234
echo %a:~-3,3%   displays    xyz
echo %a:~3%       displays   3456789abcdefghijklmnopqrstuvwxyz
echo %a:~3,-2%   displays   3456789abcdefghijklmnopqrstuvwx
echo %a:~,-2%     displays   0123456789abcdefghijklmnopqrstuvwx

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
set KL=0
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)
%~nx0=fred.g4b
%~x0=.g4b
%~n0=fred
%~dp0=(hd0,0)/xx/
%~dpnx0=(hd0,0/xx/fred.g4b
%~p0=/xx/
%~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.

Batch file to get all parameters as arg1...argn  and argc as number of parameters:

!BAT
set argc=0
:LOOP
if "%1"=="" goto :fin
set /a argc=%argc%+1 > nul
set arg%argc%=%1
shift
goto :LOOP
:fin
set

Note: BEWARE! in batch files beware of  %0  %1  %2 accidents! e.g. 

set f=3
echo %f%2

will not display 32  (it displays %f) - instead use this...

set f=3
echo %f%%2

%% in a batch file is reduced to %

Accessing numbered variables

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

!BAT
set FRED1=A
set FRED2=B
set FRED3=C
set N=0

:LOOP
set /a N=%N%+1 > nul
call echo %^FRED%N%%%
if %N%<=2 goto :LOOP

Produces:
A
B
C

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 + 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.
?_WENV
?_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:

command --set-ext=.g4b;.g4x

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.

Fn calls

You can call an internal function like sprintf within a grub4dos menu or a grub4dos batch file. For example:
set a=aa
set b=bbb
call Fn.0 0 "stringa=%s stringb=%s" %a% %b%

this prints on the display
stringa=aa stringb=bbb

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
1 grub_putstr
2 putchar - e.g. call Fn.2 49 prints '1'  (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 2 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_strtoksee here for 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
17 reserved
18 reserved
19 getkey - gets a kbdchar
20 checkkey Check if any input character is available.
21
22 grub_memcmp
23 grub_memmove
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
28 grub_close

29
30
31
32 devread
33 devwrite
34 next_partition
35 open_device
36 real_open_partition
37 set_device
38
39
40
41 parse_string
42 hexdump - e.g. call Fn.42 0x8000 0 3 - list 3 hex bytes at 0x8000
43 skip_to
44 builtin_cmd
45 get_datetime
46 find_command
47
48
49 get_mmap_entry
50 grub_malloc
51 grub_free51
52 list_partitions
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/
62 print_a_completion
63 print_completions
64 lba_to_chs
65 probe_bpb
66 probe_mbr
67 unicode_to_utf8 e.g. call Fn.67 *0x82d0 0x60000 3     cat (md)0x300+1,1 | set nt_ver=NT
68 rawread
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,
71 grub_tolower
72 grub_isspace
73 grub_sleep e.g. call Fn.73 3 to sleep for 3 secs
74 mem64
75 envi_cmd
76 strncmpx
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.
78 get_cmdline


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 possible number). 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:

Address Length Description
========= ======== ==============================================
0000:8208         4 (DWORD) install_partition (the boot partition)  (??)
0000:8274         2 (WORD)         default menu entry
0000:8276         1 (BYTE)          current selected menu entry set /a SELMENU=*0x8276 & 0xff
0000:8280         4 (DWORD) boot_drive (the boot drive)  set /A BD=*0x8280&0xf0      if %BD%==0x80 echo We booted from a hard disk!
0000:8284         4 (DWORD) pxe_yip (your ip)
0000:8288         4 (DWORD) pxe_sip (server ip)
0000:828C         4 (DWORD) pxe_gip (gateway ip)
0000:8290         8 (QWORD) filesize (file size by last "cat --length=0")
0000:8298         4 (DWORD) saved_mem_upper (extended memory size in KB)
0000:829C         4 (DWORD) saved_partition (current root partition) set /A PART=*0x829e&0xff
0000:82A0         4 (DWORD) saved_drive (current root drive) 
0000:82A4         4 (DWORD) no_decompression (no auto gunzip)  1=no decomp
0000:82A8         8 (QWORD) part_start (start sector of last partition)
0000:82B0         8 (QWORD) part_length (total sectors of last partition)
0000:82C0        8 (QWORD)       saved_mem_higher (max contiguous mem in KB starting at 4G
0000:8308        1 (BYTE)            character used for arrow symbol at left of highlighted menu entry (default=0x10 ►) - use 0 if not wanted or 0x1a for →

grub4dos 0.4.6
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 with a hyphen after the number - write 0x8274 0x2d01
0x8276 set /a CURDEF=*0x8276 & 0ff gets current menu item number as a variable CURDEF
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%


New commands August 2014

write --bytes=1 0x8277 12          - writes a single byte to address 0x8277

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:

VAR13 - filesystem type
VAR28 - graphics cursor
VAR29 - menu border character?
VAR42 - current color BG-FG as byte
VAR43 - low 32-bits foreground
VAR44 - low 32-bits background
VAR45 - cmd line string
VAR46 - splashimage loaded

Colours
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.

Other useful stuff

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
debug off
goto :%key% || echo -n

:1
:: do stuff here if no key pressed

:0x3F00 F5
:: 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
set UUID=%?_UUID%

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
set id="fred,doris"
call Fn.0 0 %id% | set id=

The  ! operator

! can only be used after a && or || operator to mean 'else'

set e=9
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

New operators (chenal grub4dos versions April 2013 and later)

The && and || operators do not affect the environment until the whole line has been executed - e.g.

set a=1 && echo %a%

does not echo '1' because the environment is not updated until after the whole line has been executed. There are now three new operators:

;;  - used to separate commands on the same line - e.g.

set a=1 ;; echo %a% ;; # This is a comment ;; set /a b=%a% + 1

;& - as ;; but the next command is only executed if the result of the previous command was true, e.g.

set a=menu.lst ;; if exist /%a% ;& echo %a% exists

;| - as ;; but next command is only executed if the previous command was false.

BEWARE - When using the write command!

The syntax of write is:

write [--offset=SKIP] ADDR_OR_FILE  INTEGER_OR_STRING

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.

e.g.
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
or
# 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
set count=%@retval%
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:

map (md)0x300+200 (rd) > nul ;; read 0x82d0 > nul ;; call Fn.24 %@retval% 0x00 102400 > nul dd if=(rd)+1 of=()/ABC.xml
# 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

Redirection

Sometimes echo will not redirect correctly to a file or memory - e.g. when it contains a $[xxxx] string...
echo $[0200]fred > (md)0x300+1
will only put 'fred' in memory and the $[0200] will not be redirected.

To circumvent this do not to use echo - e.g.

pause --wait=0 $[0200]fred > (md)x0300+1

or 
call Fn.0 0 $[0200]fred > (md)x0300+1 
write (md)0x300+1 $[0200]fred

or 
write (md)0x200+1 $[0200]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 
set A=1
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








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  1k v. 1 15 Oct 2013, 11:26 Steve Si
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