
MC68HC705J2
Rev. 2
INSTRUCTION SET
MOTOROLA
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10.2.2 Read-Modify-Write Instructions
These instructions read a memory location or a register, modify its contents, and
write the modified value back to the memory location or to the register. The test for
negative or zero instruction (TST) is an exception to the read-modify-write
sequence because it does not write a replacement value.
Table 10-2
lists the
read-modify-write instructions.
10.2.3 Jump/Branch Instructions
Jump instructions allow the CPU to interrupt the normal sequence of the program
counter. The unconditional jump instruction (JMP) and the jump to subroutine
instruction (JSR) have no register operand. Branch instructions allow the CPU to
interrupt the normal sequence of the program counter when a test condition is met.
If the test condition is not met, the branch is not performed. All branch instructions
use relative addressing.
Bit test and branch instructions cause a branch based on the state of any readable
bit in the first 256 memory locations. These three-byte instructions use a
combination of direct addressing and relative addressing. The direct address of the
byte to be tested is in the byte following the opcode. The third byte is the signed
offset byte. The CPU finds the conditional branch destination by adding the third
byte to the program counter if the specified bit tests true. The bit to be tested and
Table 10-2. Read-Modify-Write Instructions
Instruction
Mnemonic
Arithmetic Shift Left
ASL
Arithmetic Shift Right
ASR
Clear Bit in Memory
BCLR
Set Bit in Memory
BSET
Clear
CLR
Complement (One’s Complement)
COM
Decrement
DEC
Increment
INC
Logical Shift Left
LSL
Logical Shift Right
LSR
Negate (Two’s Complement)
NEG
Rotate Left through Carry Bit
ROL
Rotate Right through Carry Bit
ROR
Test for Negative or Zero
TST