HC05 datasheet(161/232 Pages) MOTOROLA

161 / 232 page

MC68HC05X16

Rev. 1

MOTOROLA

11-13

CPU CORE AND INSTRUCTION SET

11.3.8

Relative

The relative addressing mode is only used in branch instructions. In relative addressing, the

contents of the 8-bit signed byte (the offset) following the opcode are added to the PC if, and only

if, the branch conditions are true. Otherwise, control proceeds to the next instruction. The span of

relative addressing is from –126 to +129 from the opcode address. The programmer need not

calculate the offset when using the Motorola assembler, since it calculates the proper offset and

checks to see that it is within the span of the branch.

EA = PC+2+(PC+1); PC

← EA if branch taken;

otherwise EA = PC

← PC+2

11.3.9

Bit set/clear

In the bit set/clear addressing mode, the bit to be set or cleared is part of the opcode. The byte

following the opcode speciﬁes the address of the byte in which the speciﬁed bit is to be set or

cleared. Any read/write bit in the ﬁrst 256 locations of memory, including I/O, can be selectively set

or cleared with a single two-byte instruction.

EA = (PC+1); PC

← PC+2

Address bus high

← 0; Address bus low ← (PC+1)

11.3.10

Bit test and branch

The bit test and branch addressing mode is a combination of direct addressing and relative

addressing. The bit to be tested and its condition (set or clear) is included in the opcode. The

address of the byte to be tested is in the single byte immediately following the opcode byte (EA1).

The signed relative 8-bit offset in the third byte (EA2) is added to the PC if the speciﬁed bit is set

or cleared in the speciﬁed memory location. This single three-byte instruction allows the program

to branch based on the condition of any readable bit in the ﬁrst 256 locations of memory. The span

of branch is from –125 to +130 from the opcode address. The state of the tested bit is also

transferred to the carry bit of the condition code register.

EA1 = (PC+1); PC

← PC+2

Address bus high

← 0; Address bus low ← (PC+1)

EA2 = PC+3+(PC+2); PC

← EA2 if branch taken;

otherwise PC

← PC+3

159

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