Page 358 - Embedded Microprocessor Systems Real World Design

Indirect Addressing
Indirect addressing uses a register to point to the data. Continuing with our example, the
processor executes the instruction at location 0002. This is an indirect instruction that says,
“Put the value addressed (pointed to) by IAR into register RO.” Since IAR contains 0010 and
location 0010 contains 12M, we end up with the value 12AF3 in register RO.

Direct Branching
Direct branching, like direct addressing, includes the destination address (new PC value) as
part of the instruction. Our example system continues, executing the instruction at location
0003. This is a direct branch instruction that says, “Start executing at the location pointed
to by the data value following the opcode.” Because this data value is 0008, the processor
loads the PC with 0008 and continues on.

Indirect Branching
Again, like indirect addressing, indirect branching takes the destination address from a
register. In our example, the processor executes the instruction at location 0008, which loads
the IAR with a new value (0015). The instruction at location OOOA says, “Start executing at
the location addressed by IAR.” Because IAR contains 0015, the next instruction is fetched
from there.

Indexed Addressing

Indexed addressing uses two values to access a location. In the example, register RO is loaded
with 0004 by the instruction at address 0012/0013. IAR already contains 0012 from the indi-
rect branch instruction just executed. The instruction at 0014 is an indexed instruction that
says, “Load RO with the value addressed by [IAR + RO] .” Since IAR + RO = 0012 + 0004 =
0016, the value from 0016 (2AC7) is loaded into RO. Note that we loaded the value into one
of the registers used to calculate the address; we could have loaded it into another register
in the processor. Some microprocessors support an indexeddirect instruction where one of
the two index parameters is immediate data in the instruction.

Indexed Branching
Although not shown in the example, indexed branching works the same way as indexed
addressing, where a pair of registers are added to generate the destination address. A special
case of indexed direct branching commonly is used in microprocessors where a direct data
value is used as an index from the program counter. The direct data value usually is a signed
8 or 16bit number, allowing branching of +127 (%bit) or +32K (l6bit) locations. For
example, if the program counter is 12BC, a branch instruction that contains an %bit value
of 06 would cause a branch to 12C2 (12BC + 06). On some microprocessors, this is the only
kind of conditional branch available, and only unconditional branches have the ability to
reach the entire address range of the CPU.

Appendix D 339