MC68HC05P4A Datasheet, PDF (46/72 Pages) Advanced Analogic Technologies – Microcontrollers

Instruction Set
11.2.4 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 Freescale 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.
11.2.5 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.
11.2.6 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.
11.2.7 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 conditional 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. These instructions can
address any location in memory.
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 Freescale assembler determines the shortest form of indexed
addressing.
11.2.8 Relative
Relative addressing is only for branch instructions. If the branch condition is true, the CPU finds the
conditional branch destination by adding the signed byte following the opcode to the contents of the
program counter. If the branch condition is not true, the CPU goes to the next instruction. The offset is a
signed, two’s complement byte that gives a branching range of –128 to +127 bytes from the address of
the next location after the branch instruction.
When using the Freescale assembler, the programmer does not need to calculate the offset, because the
assembler determines the proper offset and verifies that it is within the span of the branch.
MC68HC05P4A Data Sheet, Rev. 7.1
46
Freescale Semiconductor