INT 15h AH=88h

INT 15h AH=88h reports the memory above the first megabyte, extended memory, in kilobytes. The 8086 can address only 1 MB, so this memory exists only on a 286 or later and is reachable in protected mode, through INT 15h AH=87h (copy a block) or through an XMS driver such as HIMEM.SYS. The value comes from the CMOS, set up by the BIOS at boot.

MOV     AH, 88h       ; BIOS: extended memory size
INT     15h           ; AX = KB above 1 MB
JC      NONE1         ; CF = 1: an 8086/8088 PC, no such function
CALL    PDEC
MOV     DX, OFFSET KB1
MOV     AH, 09h       ; DOS: print the $-string at DS:DX
INT     21h
JMP     DONE1
NONE1:
MOV     DX, OFFSET NO1
MOV     AH, 09h       ; DOS: print the $-string at DS:DX
INT     21h
DONE1:
 
KB1     DB      " KB of extended memory free for INT 15h", 13, 10, "$"
NO1     DB      "no extended memory function", 13, 10, "$"
PDEC    PROC    NEAR  ; print AX in decimal; keeps every register
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BX, 10
XOR     CX, CX        ; digits on the stack
DECD1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
TEST    AX, AX
JNZ     DECD1
DECD2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    DECD2
POP     DX
POP     CX
POP     BX
POP     AX
RET
PDEC    ENDP

Why 0 is a normal answer

The function predates memory managers. When HIMEM.SYS (or any XMS driver) loads, it takes over extended memory and hooks INT 15h so that AH=88h returns 0: an old program that would otherwise use the memory directly through AH=87h sees none and does not overwrite what the XMS driver hands out. So 0 usually means "managed by someone else", and a program that wants extended memory asks the XMS driver (INT 2Fh AX=4300h, then its entry point). DOSBox-X has an XMS driver built in and on by default, so this call returns 0 there.

Limits

AX is 16 bits, so the answer stops at 65 535 KB, just under 64 MB, and many BIOSes cap it at 15 MB or 63 MB. For more, 386 and later BIOSes offer AX=E801h and the memory map AX=E820h. On a PC or XT the function returns CF = 1, AH = 86h.