The environment block is always paragraph aligned (starts at an address that is a multiple of 16 bytes) and contains a series of ASCIIZ strings. Each of the strings takes the following form:
NAME=PARAMETER
An additional zero byte (Figure 12-3) indicates the end of the entire set of strings. Under MS-DOS version 3, the block of environment strings and the extra zero byte are followed by a word count and the complete drive, path, filename, and extension used by EXEC to load the program.
0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0000 43 4F 4D 53 50 45 43 3D 43 3A 5C 43 4F 4D 4D 41 COMSPEC=C:\COMMA
0010 4E 44 2E 43 4F 4D 00 50 52 4F 4D 50 54 3D 24 70 NDcom.PROMPT=$p
0020 24 5F 24 64 20 20 20 24 74 24 68 24 68 24 68 24 $_$d $t$h$h$h$
0030 68 24 68 24 68 20 24 71 24 71 24 67 00 50 41 54 h$h$h $q$q$g.PAT
0040 48 3D 43 3A 5C 53 59 53 54 45 4D 3B 43 3A 5C 41 H=C:\SYSTEM;C:\A
0050 53 4D 3B 43 3A 5C 57 53 3B 43 3A 5C 45 54 48 45 SM;C:\WS;C:\ETHE
0060 52 4E 45 54 3B 43 3A 5C 46 4F 52 54 48 5C 50 43 RNET;C:\FORTH\PC
0070 33 31 3B 00 00 01 00 43 3A 5C 46 4F 52 54 48 5C 31;....C:\FORTH\
0080 50 43 33 31 5C 46 4F 52 54 48 2E 43 4F 4D 00 20 PC31\FORTH.COM.
Figure 12-3. Dump of a typical environment block under MS-DOS version 3. This particular example contains the default COMSPEC parameter and two relatively complex PATH and PROMPT control strings that were set up by entries in the user's AUTOEXEC file. Note the path and file specification of the executing program following the double zeros at offset 0073H that denote the end of the environment block.
Under normal conditions, the environment block inherited by a program will contain at least three strings:
COMSPEC=variable
PATH=variable
PROMPT=variable
MS-DOS places these three strings into the environment block at system initialization, during the interpretation of SHELL, PATH, and PROMPT directives in the CONFIG.SYS and AUTOEXEC.BAT files. The strings tell the MS-DOS command interpreter, COMMAND.COM, the location of its executable file (to enable it to reload the transient portion), where to search for executable external commands or program files, and the format of the user prompt.
You can add other strings to the environment block, either interactively or in batch files, with the SET command. Transient programs can use these strings for informational purposes. For example, the Microsoft C Compiler looks in the environment block for INCLUDE, LIB, and TMP strings to tell it where to find its #include files and library files and where to build its temporary working files.