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The amount of contiguous extended memory identified by software Interrupt 15, function 88 (Int 15H/88H, get extended memory size) is the amount that HIMEM.SYS uses to allocate XMS. In versions of MS-DOS earlier than 4.01, a utility was not provided to display the amount of memory on a computer. To determine how much extended memory is installed in a computer, do the following:

• Remove any memory managers from the CONFIG.SYS file, including HIMEM.SYS.
• Reboot the computer.
• At an MS-DOS command prompt, type DEBUG.
• When the DEBUG command prompt (a hyphen, [-]) is displayed, enter the DEBUG commands below:

DEBUG       Enter DEBUG
Prompts     Commands            Comments
-------     -----------         --------

-           A 100               Assemble from CS:0100.
nnnn:0100   MOV AX, 8800        Set AX reg. for function 8800
                                (nnnn is segment address)
nnnn:0103   INT 15              Call Interrupt 15
nnnn:0105   <Press Enter>
-           P 2                 Debug Proceed instruction
                                to execute interrupt request.

Information similar to the following will appear:

AX=8800  BX=0000  CX=0000  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000
DS=1D5B  ES=1D5B  SS=1D5B  CS=1D5B  IP=0102   NV UP EI PL NZ NA PO NC
1D5B:0102 CD15          INT     15

AX=1D80  BX=0000  CX=0000  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000
DS=1D5B  ES=1D5B  SS=1D5B  CS=1D5B  IP=0104   NV UP EI PL NZ NA PO NC
1D5B:0104 7E12          JLE     0118

-           Q                        Quit Debug

The returned information is a dump of the CPU registers after each instruction. The AX register of the second dump contains the hexadecimal value for the amount of extended memory on the system. In the above example, AX=1D80. The hexadecimal value 1D80 is 7552 in decimal. Multiply the decimal number by 1024 to obtain the bytes available contiguous extended memory. In this example:

   7552 * 1024 = 7733248 bytes total contiguous extended memory