Chapter
2
I
MS-DOS
Control Blocks
and
Work Areas
Register AX indicates whether
or
not the drive specifiers (en-
tered with the
first
two parameters) are valid, as follows:
AL
=
FFH if the first parameter contained an invalid
drive specifier (otherwise, AL
=
00H).
AH
=
FFH
if the second parameter contained an invalid
drive specifier (otherwise, AH
=
00H).
Offset
2
(one word) contains the segment address of the first
byte of unavailable memory. Programs must not modify ad-
dresses beyond this point unless the addresses were obtained
by allocating memory via the Allocate Memory function call
(48H).
For
executable (.EXE) programs:
0
Registers DS and
ES
are set
to
point
to
the Program Segment
0
Registers CS,
IP,
SS,
and
SP
are set
to
the values passed by
For
executable (.COM) programs:
0
All four segment registers contain the segment address of the
initial allocation block that starts with the Program Segment
Prefix control block.
0
All of user memory
is
allocated
to
the program. If the pro-
gram invokes another program through the EXEC function
call (4BH),
it
must
first
free some memory through the Set
Block function call (4AH)
to
provide space for the program
being executed.
0
The Instruction Pointer (IP)
is
set
to
100H.
The Stack Pointer register is set
to
the end
of
the program’s
segment. The segment size at offset
6
is reduced by lOOH
to
allow for
a
stack of that size.
0
A word of zeroes
is
placed on top of the stack. This is
to
allow
a
user program
to
exit
to
COMMAND.COM by doing a RET
instruction last. This assumes the user has maintained stack
and code segments for the program.
Prefix.
MS-LINK.
156
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