68HC705SP7 datasheet(190/230 Pages) FREESCALE

190 / 230 page

Instruction Set

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Instruction Set

14.3.5 Indexed, No Offset

Indexed instructions with no offset are 1-byte instructions that can

access data with variable addresses within the first 256 memory

locations. The index register contains the low byte of the effective

address of the operand. The CPU automatically uses $00 as the high

byte, so these instructions can address locations $0000–$00FF.

Indexed, no offset instructions are often used to move a pointer through

a table or to hold the address of a frequently used RAM or I/O location.

14.3.6 Indexed, 8-Bit Offset

Indexed, 8-bit offset instructions are 2-byte instructions that can access

data with variable addresses within the first 511 memory locations. The

CPU adds the unsigned byte in the index register to the unsigned byte

following the opcode. The sum is the effective address of the operand.

These instructions can access locations $0000–$01FE.

Indexed 8-bit offset instructions are useful for selecting the kth element

in an n-element table. The table can begin anywhere within the first 256

memory locations and could extend as far as location 510 ($01FE). The

k value is typically in the index register, and the address of the beginning

of the table is in the byte following the opcode.

14.3.7 Indexed,16-Bit Offset

Indexed, 16-bit offset instructions are 3-byte instructions that can access

data with variable addresses at any location in memory. The CPU adds

the unsigned byte in the index register to the two unsigned bytes

following the opcode. The sum is the effective address of the operand.

The first byte after the opcode is the high byte of the 16-bit offset; the

second byte is the low byte of the offset.

Indexed, 16-bit offset instructions are useful for selecting the kth element

in an n-element table anywhere in memory.

As with direct and extended addressing, the Motorola assembler

determines the shortest form of indexed addressing.

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68HC705SP7 데이터시트(PDF) 190 Page - Freescale Semiconductor, Inc