Representation of Negative Number:

As  mentioned  in chapter 1 in 8085  negative  numbers are represented in 2 complement  form  and subtraction is  performed  using 2  complemented  method.  If the  result of subtraction is negative  it will also  be in 2 complement  form. But  D₇ ( left most bit) is used  to indicate sign  of a number. If D₇  is 0 the number  is positive  and if D₇  is 1  the number is negative D₇  is 1 for  2 complement of  all D₇ bit  numbers.

Important : Generally  people  have misconnect that the  difference  between a positive  number and a  negative  number is only in  D₇.  They assume that if  +5 ₁₀ is  written  in 8 bit  binary as 00001012 then -5₁₀  can be  written  as 10000102, which  is not  correct.

Refer followings  table  showing various  positive  and negative numbers to understand the concept  of negative  number as used  in microprocessors.

a.All negative numbers have 1 in  their left  most bit positions.

b.All negative  numbers are 2  complement  form  of their  corresponding  positive  numbers.

c. All negative  have their  equivalent positive  decimal number. It is up to the programmer  and the  application that whether  to use it as  negative or positive number.

d.If two numbers are added/ subtracted the  result will be as desired whether is a singed  or unsigned operation. See  followings  example.