Determining Load Size of Device Drivers or TSRs (74985)

The information in this article applies to:
    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 Q74985


Three size factors determine the space needed to load a device driver or TSR (terminate-and-stay-resident) program into the upper memory area (UMA). The most important of these factors is the size that must be available in one contiguous upper memory block (UMB) for the device driver or TSR to load high. These factors are as follows:

Load Size

The size of the device/TSR when it's first loaded, before it begins execution. Frequently this is the size of the file on the disk, but may be larger.

Initialization Size

The size of the device/TSR and any initialization space it may need. Frequently the initialization size is equal to load size, but it may be the largest of the three sizes.

Final Size

The amount of memory needed after the completion of the initialization process. Frequently the smallest of the three sizes. This is the size indicated by the MEM command using the /P or /C switches.


Because of the lack of standards for device driver and TSR initialization, it is virtually impossible to determine the memory needed in any other manner than trial and error. Load size can be determined; however, initialization and final sizes will vary from program to program.

Quantity 1

For .COM files and device drivers that follow standard conventions (.SYS), the load size will be the file size on disk rounded up to the nearest paragraph (16 bytes). Note that some .SYS files will actually be .EXE files. To determine this, look at the first two bytes of the file. If these bytes are MZ in ASCII or 4D and 5A in hex, it is an executable file.

Executable files (.EXE) files contain the load size information in the program header (.EXE file header). The .EXE file load size does not necessarily relate to the file size on disk. At an offset of 10 and 12 bytes respectively reside the minimum and maximum number of pages (16 bytes) needed by the executable file in addition to the file size. EXEHDR.EXE included with many programming products (including Microsoft C Compiler) will allow you to look at the .EXE file header information. From this information you will be able to approximate the load size of an .EXE program.

For example, by running EXEHDR.EXE included with Microsoft C version 6.0a on EMM386.EXE, the following information will be displayed:
Microsoft (R) EXE File Header Utility  Version 2.01
Copyright (C) Microsoft Corp 1985-1990.  All rights reserved.

Library:                        LoadHi
Description:                    Win386 LoadHi Device  (Version 1.0)
Module type:                    Dynamic link library; initialization
                                NO external fixups in executable image
Number of memory pages:         00000005 (5)
Initial CS:EIP:                 object 3 offset 00000000
Initial SS:ESP:                 object 0 offset 00000000
Automatic data object:          0

 no. virtual  virtual  page     file     flags
      address   size    map      pages
0001 00000000 00001a08 00000001 00000002 EXECUTABLE, READABLE,
                                PRELOAD, 32-bit
0002 00002000 00001218 00000003 00000002 EXECUTABLE, READABLE,
                                         DISCARDABLE, 32-bit
0003 00004000 0000002a 00000005 00000001 EXECUTABLE, READABLE,
                                         16:16 ALIAS

ord obj  offset    name
  1   1  000012e4  LoadHi_DDB exported, shared data
By totaling the virtual sizes of 1a08H, 1218H, and 02AH you can determine the load size to be a total of 11338 decimal. Note that this is not necessarily the ending memory used by EMM386.EXE, but the initial load size.

Quantity 2

Many device drivers undergo an initialization process that normally requires more memory than the ending size of the driver. Frequently the initialization memory is placed after the device driver and subsequently freed upon completion of the initialization process. The method for doing so is not standardized, and therefore not consistent. For this reason, there is no way of knowing what method a driver developer will use, and therefore no way of determining how much memory will be used, other than by actually disassembling the code and working through the program.

Quantity 3

The final quantity, the Break address of the driver, is returned during the INIT procedure of a device driver or the termination call of a TSR.

When loading a device driver, DOS will pass the device driver a command code 0 for initialization. It is the device driver's responsibility to fill the Request Header (a DOS data structure) with the break address (ending address) of the device driver. When and where the code is located that the device driver uses to accomplish this task is completely up to the programmer. For this reason, the only way of determining the break address is to debug the device driver.

A TSR program will make a DOS function call to one of the following functions:
   Int 21h function 00h
   Int 21h function 31h
   Int 21h function 4Ch
   Int 27h
The program will provide DOS with the break address of the TSR via different methods depending upon the function used.


"Microsoft MS-DOS Encyclopedia," pages 450-60
"The Programmer's PC Sourcebook" by Thom Hogan
"Microsoft Systems Journal," vol. 6, no. 4, July 1991
"MS-DOS Functions" by Ray Duncan

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB74985