Freescale Semiconductor, Inc.
GENERAL RELEASE SPECIFICATION
12.2.3 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 3-byte instructions that use direct
addressing to access the operand and relative addressing to specify a branch
destination.
12.2.4 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.
12.2.5 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 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.
12.2.6 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 conditional 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.
INSTRUCTION SET
MC68HC805P18
12-2
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