Question

If a BRA instruction is located at $01CD, what is the maximum backward destination address

Answer 1

Calculating the Destination Address

In the relative mode, a relative address follows the opcode in memory. It is added to the current program counter to determine the destination address. The relative address is a signed 8-bit number that indicates the number of bytes to branch forward or backward in memory to the destination address. It is a displacement from the current position in memory. Since it is a signed 8-bit number, it must be sign-extended to 16 bits before being added to the contents of the 16-bit program counter.

DA = PC + rr

DA = Destination Address

PC = Address in the Program Counter

rr = Sign-Extended Relative Address

DA = $01 + 2 -> $03

rr = $CD -> $CD

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DA ------> $92