Motorola DSP56000 Address Register Indirect Modes, No Update, Postincrement By, Postdecrement By

ADDRESSING

the AGU and are described in the following paragraphs.

4.4.1 Address Register Indirect Modes

When an address register is used to point to a memory location, the addressing mode is called “address register indirect” (see Table 4-1). The term indirect is used because the register contents are not the operand itself, but rather the address of the operand. These addressing modes specify that an operand is in memory and specify the effective address of that operand.

A portion of the data bus movement field in the instruction specifies the memory space to be referenced. The contents of specific AGU registers that determine the effective address are modified by arithmetic operations performed in the AGU. The type of address arithmetic used is specified by the address modifier register, Mn. The offset reg- ister, Nn, is only used when the update specifies an offset.

Not all possible combinations are available, such as + (Rn). The 24-bit instruction word size is not large enough to allow a completely orthogonal instruction set for all instruc- tions used by the DSP.

An example and description of each mode is given in the following paragraphs. SEC- TION 6 - INSTRUCTION SET INTRODUCTION and APPENDIX A - INSTRUCTION SET DETAILS give a complete description of the instruction syntax used in these examples. In particular, XY: memory references refer to instructions in which an operand in X mem- ory and an operand in Y memory are referenced in the same instruction.

4.4.1.1No Update

The address of the operand is in the address register, Rn (see Table 4-1). The contents of the Rn register are unchanged by executing the instruction. Figure 4-4shows a MOVE instruction using address register indirect addressing with no update. This mode can be used for making XY: memory references. This mode does not use Nn or Mn registers.

4.4.1.2Postincrement By 1

The address of the operand is in the address register, Rn (see Table 4-1and Figure 4-5). After the operand address is used, it is incremented by 1 and stored in the same address register. This mode can be used for making XY: memory references and for modifying the contents of Rn without an associated data move.

4.4.1.3Postdecrement By 1

The address of the operand is in the address register, Rn (see Table 4-1and Figure 4-6). After the operand address is used, it is decremented by 1 and stored in the same address register. This mode can be used for making XY: memory references and for

MOTOROLA ADDRESS GENERATION UNIT 4 - 9