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The operand field operand(s) R specifies the registers used for execution. It is sometimes called register (direct) addressing, the term “direct” indicating that the referencing in the register is found in the instruction coding, as for the direct memory address (Figure 1.9). These registers are accessible to the programmer. There is no effective address since the memory is not addressed, hence a fast execution of the instruction using it and a small instruction format. It is for this reason that RISC microprocessors prefer to use this mode. For other architectures, the number of registers accessible to the programmer is limited (order of size: about 20).

Figure 1.9. Execution of an instruction using register addressing from one register

The example below shows an addition in an Arm^® microprocessor, which uses three registers: r0, r1 and r2:

ADD r0,r1,r2; r0 ← r1 + r2

Note, a distinction can be made between Data Register Direct Addressing and Address Register Direct Addressing as for MC68000 (cf. § V3-3.1.1).