HCS12 datasheet(32/452 Pages) FREESCALE

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S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

In this example, extended addressing mode is used to access the operand

FOO, immediate addressing mode is used to access the mask value $03,

and relative addressing mode is used to identify the destination address of

a branch in case the branch-taken conditions are met. BRSET is listed as

an extended mode instruction even though immediate and relative modes

are also used.

3.6 Direct Addressing Mode

This addressing mode is sometimes called zero-page addressing because

it is used to access operands in the address range $0000 through $00FF.

Since these addresses always begin with $00, only the eight low-order bits

of the address need to be included in the instruction, which saves program

space and execution time. A system can be optimized by placing the most

commonly accessed data in this area of memory. The eight low-order bits of

the operand address are supplied with the instruction, and the eight

high-order bits of the address are assumed to be 0.

Example:

LDAA

$55

This is a basic example of direct addressing. The value $55 is taken to be

the low-order half of an address in the range $0000 through $00FF. The

high order half of the address is assumed to be 0. During execution of this

instruction, the CPU combines the value $55 from the instruction with the

assumed value of $00 to form the address $0055, which is then used to

access the data to be loaded into accumulator A.

Example:

LDX

$20

In this example, the value $20 is combined with the assumed value of $00

to form the address $0020. Since the LDX instruction requires a 16-bit

value, a 16-bit word of data is read from addresses $0020 and $0021. After

execution of this instruction, the X index register will have the value from

address $0020 in its high-order half and the value from address $0021 in its

low-order half.

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