76h UT$SMUL signed integer multiplication
Input X register - Signed integer.
D register - Signed integer.
Output X register - Less significant word of product.
D register - More significant word of product.
Description

2-byte by 2-byte signed integer multiply routine.

Multiplies D by X putting the product back into D and X. The more significant word of the product goes into D.

Example
        LDX     SIGNED_MULTIPLIER
        LDD     SIGNED_MULTIPLICAND
        OS      UT$SMUL
        TSTA                  ; test the most significant byte
        BMI     NEGATIVE_PRODUCT
Errors - none -