How to Use the STACKS= Setting in the CONFIG.SYS File (84300)
The information in this article applies to:
• Microsoft MS-DOS operating system 3.1
• Microsoft MS-DOS operating system 3.2
• Microsoft MS-DOS operating system 3.21
• Microsoft MS-DOS operating system 3.3
• Microsoft MS-DOS operating system 3.3a
• Microsoft MS-DOS operating system 4.0
• Microsoft MS-DOS operating system 4.01
• Microsoft MS-DOS operating system 5.0
• Microsoft MS-DOS operating system 5.0a
• Microsoft MS-DOS operating system 6.0
• Microsoft MS-DOS operating system 6.2
• Microsoft MS-DOS operating system 6.21
• Microsoft MS-DOS operating system 6.22
This article was previously published under Q84300
Mysterious hangs or failures can occur when a hardware routine does
not have enough storage space to perform its job. To avoid this
problem, MS-DOS versions 3.2 and later provide additional storage
space for the interrupt-handling code whenever a hardware interrupt
occurs. This is done by changing the current storage area (called a
"stack," because it is a place to stack information temporarily) to a
new storage area provided by MS-DOS.
By default, nine of these stacks, each 128 bytes in size, are
provided. The number and size of stacks can be increased using the
CONFIG.SYS STACKS=n,m command, where n is the number of stacks and m
is the size of each stack in bytes.
Changing the current stack takes time. It was discovered that
changing the stack can slow the system enough that some programs,
particularly high-speed communications programs, will fail. To avoid
this problem, MS-DOS versions 3.3 and later allow disabling the stack
swapping by specifying 0 stacks of 0 bytes in size (STACKS=0,0).
Because the STACKS= command affects hardware- and
configuration-dependent code, it can have unpredictable effects.
• If a "stack overflow" or "exception error 12" (stack exception)
error occurs, increasing the number and size of stacks specified in
CONFIG.SYS may solve the problem (for example, STACKS=18,256).
Note: STACKS= only affects hardware interrupts. If the stack being
overflowed (trying to store too much information) is internal to an
application, this will probably not solve the problem.
• If the command STACKS=0,0 is in the CONFIG.SYS file and any
instability occurs (including, but not limited to, printing
problems and system hangs), remove the STACKS= command
temporarily and test.
• If the command STACKS=n,m is in CONFIG.SYS and problems occur
(including, but not limited to, serial port problems), remove it
temporarily and test. If problems continue and the system is an AT
class machine, set STACKS=0,0; if the system is a PC, PC XT, or PC
Convertible, set STACKS=9,128.
Beginning in MS-DOS version 3.2, MS-DOS added code to swap the stack
whenever an asynchronous hardware interrupt occurs, before passing
control to the original interrupt handler. (Interrupt vectors 02H,
08-0EH, 70H, and 72-77H, corresponding to IRQs 0-15.) Before MS-DOS
3.2, interrupt handlers received control with the stack set to that of
whatever program or device driver was executing when the interrupt
occurred. Thus, if a program stack is nearly full and a hardware
interrupt occurs, random and unpredictable behavior will result
(typically a system hang).
Beginning in MS-DOS 3.3, the STACKS=0,0 line was added. This setting
disables MS-DOS stack switching (thus returning MS-DOS stack behavior
to that of MS-DOS versions 3.1 and earlier). That is, interrupts are
processed using whichever stack happens to be loaded at the time of
the interrupt. Because MS-DOS doesn't manage any stacks, less memory
is required when 0,0 is specified. In addition, less time is required
to handle interrupts because no code to swap stacks is executed. A
negative aspect is that the same behavior that motivated support for
stack swapping can occur (that is, system hangs and other random
behavior as the stack overflows).
If stack switching is enabled, MS-DOS ensures nested interrupts do not
get the same stack. Upon completion of the interrupt processing, the
stack is released so that MS-DOS may use it for a future interrupt.
Ranges for n and m in STACKS=n,m
• MS-DOS 3.1 and earlier have no support for swapping of stacks
• MS-DOS 3.2 and later support values in range N=8-64, M=32-512
• MS-DOS 3.3 added support for 0,0, which is equivalent to behavior
in 3.1 and earlier.
Default Values for STACKS
• MS-DOS versions earlier than 3.2 do not support STACKS; behavior
is equivalent to STACKS=0,0 on MS-DOS 3.3 and later.
• MS-DOS 3.2 always defaults to STACKS=9,128.
• MS-DOS 3.3 and later use special case codes that check for PC,
PC XT, and PC Convertible. If any of these is detected, STACKS
defaults to 0,0. Otherwise, STACKS defaults to 9,128.
If a system is experiencing random hangs or strange behavior and
STACKS=0,0 is in the CONFIG.SYS file, this should be considered a
prime suspect and removed. Conversely, some hardware that requires
extremely fast interrupt processing may require STACKS=0,0 to operate
properly. If you are having trouble with a high-speed communications
or real-time program, try setting STACKS=0,0. If you have a program
that worked correctly with MS-DOS 3.1 or earlier and you cannot get
it to work with MS-DOS 3.2 or later, set STACKS=0,0.
If stack switching is enabled and MS-DOS runs out of free stacks, the
following message will be displayed:
If the above message is displayed, try a higher number for n in
STACKS=n,m. If you experience random hangs or other strange behavior,
try increasing the m value in STACKS=n,m.
If you are hooking interrupts with the MS-DOS get/set interrupt vector
services, stack switching will not be done before your interrupt
handler gets control. Any stack switching necessary must be done by
your interrupt handler.
"Microsoft MS-DOS Encyclopedia," page 805
"Microsoft MS-DOS User's Guide and Reference," version 5.0 pages 250,
284, and 570
"Microsoft Systems Journal," July 1991; Vol. 6, No 4; pages 25-26
"Microsoft MS-DOS Programmer's Reference," version 5.0, pages 109-110
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