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;* ImulLong - Multiplies two signed double-word integers. Because the imul
;* instruction (illustrated here) treats each word as a signed number, its
;* use is impractical when multiplying multi-word values. Thus the technique
;* used in the MulLong procedure can't be adopted here. Instead, ImulLong
;* is broken into three sections arranged in ascending order of computational
;* overhead. The procedure tests the values of the two integers and selects
;* the section that involves the minimum required effort to multiply them.
;*
;* Shows:   Instruction - imul
;*
;* Params:  long1 - First integer (multiplicand)
;*          long2 - Second integer (multiplier)
;*
;* Return:  Result as long integer
 
ImulLong PROC \
        USES si, \
        long1:DWORD, long2:DWORD
 
; Section 1 tests for integers in the range of 0 to 65,535. If both
; numbers are within these limits, they're treated as unsigned short
; integers.
 
sect1:  mov     ax, WORD PTR long2[0]   ; AX = low word of long2
        mov     dx, WORD PTR long2[2]   ; DX = high word of long2
        mov     bx, WORD PTR long1[0]   ; BX = low word of long1
        mov     cx, WORD PTR long1[2]   ; CX = high word of long1
        or      dx, dx                  ; Both high words zero?
        jnz     sect2                   ; No?  Go to section 2
        or      cx, cx
        jnz     sect2
        mul     bx                      ; Yes?  Multiply the low words
        jmp     SHORT exit              ;   and exit section 1
 
; Section 2 tests for integers in the range of -32,768 to 32,767. If
; both numbers are within these limits, they're treated as signed short
; integers.
 
sect2:  push    ax                      ; Save long2 low word
        push    bx                      ; Save long1 low word
        or      dx, dx                  ; High word of long2 = 0?
        jnz     @F                      ; No?  Test for negative
        test    ah, 80h                 ; Low word of long2 in range?
        jz      skip1                   ; Yes?  long2 ok, so test long1
        jmp     SHORT sect3             ; No?  Go to section 3
@@:     cmp     dx, 0FFFFh              ; Empty with sign flag set?
        jne     sect3                   ; No?  Go to section 3
        test    ah, 80h                 ; High bit set in low word?
        jz      sect3                   ; No?  Low word is too high
 
skip1:  or      cx, cx                  ; High word of long1 = 0?
        jnz     @F                      ; No?  Test for negative
        test    bh, 80h                 ; Low word of long1 in range?
        jz      skip2                   ; Yes?  long1 ok, so use sect 2
        jmp     SHORT sect3             ; No?  Go to section 3
@@:     cmp     cx, 0FFFFh              ; Empty with sign flag set?
        jne     sect3                   ; No?  Go to section 3
        test    bh, 80h                 ; High bit set in low word?
        jz      sect3                   ; No?  Low word is too high
 
skip2:  imul    bx                      ; Multiply low words
        pop     bx                      ; Clean stack
        pop     bx
        jmp     SHORT exit              ; Exit section 2
 
; Section 3 involves the most computational overhead. It treats the two
; numbers as signed long (double-word) integers.
 
sect3:  pop     bx                      ; Recover long1 low word
        pop     ax                      ; Recover long2 low word
        mov     si, dx                  ; SI = long2 high word
        push    ax                      ; Save long2 low word
        mul     cx                      ; long1 high word x long2 low word
        mov     cx, ax                  ; Accumulate products in CX
        mov     ax, bx                  ; AX = low word of long1
        mul     si                      ; Multiply by long2 high word
        add     cx, ax                  ; Add to previous product
        pop     ax                      ; Recover long2 low word
        mul     bx                      ; Multiply by long1 low word
        add     dx, cx                  ; Add to product high word
 
exit:   ret                             ; Return result as DX:AX
 
ImulLong ENDP
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