SUB Subtract Instruction

This  instruction  is used to  subtract the contents of any  register  or memory location from  the contents of  accumulator. There are two  formats as follows:

a.Subtract the contents of Register The  contents of  register R are subtracted  from the contents of accumulator and  result of operation is  stored in the accumulator. Here register R  may be any  one  of the A, B, C,D, E,H or L registers.

SUBR                   ( A- A - R)

Flags : All  flags  are modified according to the results.

As we have already  discussed in chapter 1 that the subtraction is done  using 2 s  complement  method in microprocessor.  Followings  steps are followed  by the  8085 microprocessor while performing  subtract operations.

 Step1.  Converts subtrahend ( the  number to  be subtracted) into its 1 s complement form.

 Steps2.  Adds 1 to  the 1 complements  form to  obtain 2  complement  form the  subtrahend.

 Step3.  Adds 2 complements  to the minuend ( the  contents of the accumulator).

 Step 4.  Complements  the carry  flag.