HC05 datasheet(159/232 Pages) MOTOROLA

159 / 232 page

MC68HC05X16

Rev. 1

MOTOROLA

11-11

CPU CORE AND INSTRUCTION SET

11.3

Addressing modes

Ten different addressing modes provide programmers with the ﬂexibility to optimize their code for

all situations. The various indexed addressing modes make it possible to locate data tables, code

conversion tables and scaling tables anywhere in the memory space. Short indexed accesses are

single byte instructions; the longest instructions (three bytes) enable access to tables throughout

memory. Short absolute (direct) and long absolute (extended) addressing are also included. One

or two byte direct addressing instructions access all data bytes in most applications. Extended

addressing permits jump instructions to reach all memory locations.

The term ‘effective address’ (EA) is used in describing the various addressing modes. The effective

address is deﬁned as the address from which the argument for an instruction is fetched or stored.

The ten addressing modes of the processor are described below. Parentheses are used to indicate

‘contents of’ the location or register referred to. For example, (PC) indicates the contents of the

location pointed to by the PC (program counter). An arrow indicates ‘is replaced by’ and a colon

indicates concatenation of two bytes. For additional details and graphical illustrations, refer to the

M6805 HMOS/M146805 CMOS Family Microcomputer/ Microprocessor User's Manual or to

the

M68HC05 Applications Guide.

11.3.1

Inherent

In the inherent addressing mode, all the information necessary to execute the instruction is

contained in the opcode. Operations specifying only the index register or accumulator, as well as

the control instruction, with no other arguments are included in this mode. These instructions are

one byte long.

11.3.2

Immediate

In the immediate addressing mode, the operand is contained in the byte immediately following the

opcode. The immediate addressing mode is used to access constants that do not change during

program execution (e.g. a constant used to initialize a loop counter).

EA = PC+1; PC

← PC+2

11.3.3

Direct

In the direct addressing mode, the effective address of the argument is contained in a single byte

following the opcode byte. Direct addressing allows the user to directly address the lowest 256

bytes in memory with a single two-byte instruction.

EA = (PC+1); PC

← PC+2

Address bus high

← 0; Address bus low ← (PC+1)

157

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