STC12LE5616AD datasheet(82/377 Pages) STC

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Chapter 5. Instruction System

Some instructions are specific to a certain register. For example, some instructions always operate on the

accumulator or data pointer,etc. No address byte is needed for such instructions. The opcode itself does it.

Index Addressing

Only program memory can be accessed with indexed addressing and it can only be read. This addressing mode is

intended for reading look-up tables in program memory. A 16-bit base register(either DPTR or PC) points to the

base

of the table, and the accumulator is set up with the table entry number. Another type of indexed

addressing is used in the conditional jump instruction.

In conditional jump, the destination address is computed as the sum of the base pointer and the

accumulator.

5.1 Addressing Modes

Addressing modes are an integral part of each computer's instruction set. They allow specifyng the source or

destination of data in different ways, depending on the programming situation. There are five modes available:

Immediate

Direct

Indirect

Indexed

Immediate Constant(IMM)

The value of a constant can follow the opcode in the program memory. For example,

MOV

A, #70H

loads the Accumulator with the hex digits 70. The same number could be specified in decimal number as 112.

Direct Addressing(DIR)

In direct addressing the operand is specified by an 8-bit address field in the instruction. Only 128 lowest bytes of

internal data RAM and SFRs can be direct addressed.

Indirect Addressing(IND)

In indirect addressing the instruction specified a register which contains the address of the operand. Both internal

and external RAM can be indirectly addressed.

The address register for 8-bit addresses can be R0 or R1 of the selected bank, or the Stack Pointer.

The address register for 16-bit addresses can only be the 16-bit data pointer register – DPTR.

The register banks, containing registers R0 through R7, can be accessed by certain instructions which carry a 3-bit

efficient because this mode eliminates the need of an extra address byte. When such instruction is executed, one

of the eight registers in the selected bank is accessed.

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