68HC05P9A datasheet(62/90 Pages) FREESCALE

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GENERAL RELEASE SPECIFICATION

INSTRUCTION SET

MC68HC05P9A

12-2

Rev. 2.0

12.1.2 Immediate

Immediate instructions are those that contain a value to be used in an operation

with the value in the accumulator or index register. Immediate instructions require

no memory address and are two bytes long. The opcode is the ﬁrst byte, and the

immediate data value is the second byte.

12.1.3 Direct

Direct instructions can access any of the ﬁrst 256 memory addresses with two

bytes. The ﬁrst byte is the opcode, and the second is the low byte of the operand

address. In direct addressing, the CPU automatically uses $00 as the high byte of

the operand address. BRSET and BRCLR are three-byte instructions that use

direct addressing to access the operand and relative addressing to specify a

branch destination.

12.1.4 Extended

Extended instructions use only three bytes to access any address in memory. The

ﬁrst byte is the opcode; the second and third bytes are the high and low bytes of

the operand address.

When using the Motorola assembler, the programmer does not need to specify

whether an instruction is direct or extended. The assembler automatically selects

the shortest form of the instruction.

12.1.5 Indexed, No Offset

Indexed instructions with no offset are one-byte instructions that can access data

with variable addresses within the ﬁrst 256 memory locations. The index register

contains the low byte of the conditional 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.

12.1.6 Indexed, 8-Bit Offset

Indexed, 8-bit offset instructions are two-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 conditional address of the operand. These instructions can access

locations $0000–$01FE.

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