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SDA 30C163-2

2.1.3 Addressing Modes

There are five general addressing modes operating on bytes. One of these five addressing modes,

however, operates on both bytes and bits:

– register

– direct (both bytes and bits)

– register indirect

– immediate

– base-register plus index-register indirect

The following table summarizes, which memory spaces may be accessed by each of the

addressing modes:

Register Addressing

R0 – R7

ACC, B, CY (bit), DPTR

Direct Addressing

RAM (low part)

Special Function Registers

Register-Indirect Addressing

RAM (@R1, @R0, SP)

Immediate Addressing

Program Memory

Base-Register plus Index-Register Indirect Addressing

Program Memory (@DPTR + A, @PC + A)

Register Addressing

Register addressing accesses the eight working registers (R0 – R7) of the selected register bank.

The PSW-register flags RS1 and RS0 determine which register bank is enabled. The least

significant three bits of the instruction opcode indicate which register is to be used. ACC, B, DPTR

and CY, the Boolean processor accumulator, can also be addressed as registers.

Direct Addressing

Direct byte addressing specifies an on-chip RAM-location (only low part) or a special function

register. Direct addressing is the only method of accessing the special function registers. An

additional byte is appended to the instruction opcode to provide the memory location address. The

highest-order bit of this byte selects one of two groups of addresses: values between 0 and 127

(00H – 7FH) access internal RAM-locations, while values between 128 and 255 (80H – 0FFH)

access one of the special function registers.

Register-Indirect Addressing

Register-indirect addressing uses the contents of either R0 or R1 (in the selected register bank) as

a pointer to locations in the 256 bytes of internal RAM. Note that the special function registers are

not accessable by this method.

Semiconductor Group

17

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