这是我自己写的，希望对你有所帮助
MOV A,R6
MOV B,R4
MUL AB
MOV R0,A
MOV R1,B

MOV A,R7
MOV B,R4
MUL AB
ADD A,R1
MOV R1,A
MOV R2,B

MOV A,R6
MOV B,R5
MUL AB
ADD A,R1
MOV R1,A
CLR A
ADDC A,B
ADDC A,R2
MOV R2,A


MOV A,R7
MOV B,R5
MUL AB
ADD A,R2
MOV R2,A
MOV R3,B
END

这是我在学校老师那里拿来的收藏好东西 你参考下
（５） 标号： ＭＵＬＤ    功能：双字节二进制无符号数乘法   入口条件：被乘数在R2、R3中，乘数在R6、R7中。 出口信息：乘积在R2、R3、R4、R5中。 影响资源：PSW、A、B、R2～R7    堆栈需求： ２字节   MULD:          MOV             A,R3             ;计算R3乘R7                      MOV             B,R7                      MUL             AB                      MOV             R4,B             ;暂存部分积                      MOV             R5,A                      MOV             A,R3             ;计算R3乘R6                      MOV             B,R6                      MUL             AB                      ADD             A,R4             ;累加部分积                      MOV             R4,A                      CLR                     A                      ADDC           A,B                      MOV             R3,A                      MOV             A,R2             ;计算R2乘R7                      MOV             B,R7                      MUL             AB                      ADD             A,R4             ;累加部分积                      MOV             R4,A                      MOV             A,R3                      ADDC           A,B                      MOV             R3,A                      CLR                     A                      RLC                     A                      XCH             A,R2             ;计算R2乘R6                      MOV             B,R6                      MUL             AB                      ADD             A,R3             ;累加部分积                      MOV             R3,A                      MOV             A,R2                      ADDC           A,B                      MOV             R2,A                      RET