Write a segment of main program and show its subroutine structure to perform the following operations. The program is to check the three most significant bits in register DX and depending on their setting, execute one of three subroutines: SUBA, SUBB, or SUBC. The subroutines are selected as follows: a) If bit 15 of DX is set, initiate SUBA. b) If bit 14 of DX is set and bit 15 is not set, initiate SUBB. c) If bit 13 of DX is set and bits 14 and 15 are not set, initiate SUBC. If a subroutine is executed, the corresponding bit(s) of DX should be cleared and then control returned to the main program