Instruction Set Architecture and Design - Fundamentals of Computer Organization and Architecture - page 25

Autodecrement Mode Similar to the autoincrement, the autodecrement mode

uses a register to hold the address of the operand. However, in this case the
content of the autodecrement register is first decremented and the new content
is used as the effective address of the operand. In order to reflect the fact that the
content of the autodecrement register is decremented before accessing the operand,
a( ) is included before the indirection parentheses. Consider, for example, the

instruction LOAD

2

(R auto ), R i . This instruction decrements the content of the
register R auto and then uses the new content as the effective address of the operand
that is to be loaded into register R i . Figure 2.11 illustrates the autodecrement addres-
sing mode.
The seven addressing modes presented above are summarized in Table 2.2. In
each case, the table shows the name of the addressing mode, its definition, and a gen-
eric example illustrating the use of such mode.
In presenting the different addressing modes we have used the load instruction
for illustration. However, it should be understood that there are other types of
instructions in a given machine. In the following section we elaborate on the differ-
ent types of instructions that typically constitute the instruction set of a given
machine.

Figure 2.11 Illustration of the autodecrement addressing mode