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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

Figure 4.8
Instruction Byte
Arrangement

FPO

in the present example, the largest forward branch is 127 memory locations and
the largest backward branch is 128 memory locations.

Offset Example
Suppose the program counter is at address 5,122 and the instruction at
this location is a branch instruction. The instruction to which the branch is to
be made is located at memory address 5,218. Since the second byte of the
branch instruction is only 8 bits wide, the actual address 5,218 cannot be
contained therein. Therefore, the difference or offset (96) between the current
program counter value (5,122) and the desired new address (5,218) is
contained in the second byte of the branch instruction. The offset value (96) is
added to the address in the program counter (5,122) to obtain the new address
(5,218), which is then placed on the address bus. The binary computation of
the final address from the program counter value and second byte of the branch
instruction is shown in Figure 4.9
Jump Instruction

8-bit branch operations Branch instructions have a range of +127 or –128 (in the present 8-bit
are limited to an offset example). If the branch needs to go beyond this range, a jump instruction must
range of +127 or –128 be used. The jump instruction is a 3-byte instruction. The first byte is the jump
memory locations. Thus, op code, and the next two bytes are the actual jump address. The CPU loads
program branches to the jump address directly into the program counter, and the program counter
locations farther away effectively gets restarted at the new jump location. The CPU continues to fetch
must use jump instruc- and execute instructions in exactly the same way it did before the jump was
tions. These 3-byte made.
instructions contain the The jump instruction causes the CPU to jump out of one section of the
entire memory address. program into another. The CPU cannot automatically return to the first section
because no record was kept of the previous location. However, another
instruction, the jump-to-subroutine, does leave a record of the previous
instruction address.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 113