Relative; Bit Set/Clear; Bit Test And Branch - Motorola MC68HC05T16 Technical Data Manual

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.
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.
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.
Address bus high ← 0; Address bus low ← (PC+1)
MC68HC05L1
EA = PC+2+(PC+1); PC ← EA if branch taken;
otherwise EA = PC ← PC+2
EA = (PC+1); PC ← PC+2
EA1 = (PC+1); PC ← PC+2
EA2 = PC+3+(PC+2); PC ← EA2 if branch taken;
otherwise PC ← PC+3
CPU CORE AND INSTRUCTION SET
11
TPG
MOTOROLA
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