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.