Texas Instruments MSC1210 manual Conditional Branching, Direct Jumps

Description

6.1 Description

When the MSC1210 is first initialized the PC SFR is cleared to 0000H. The part then begins to execute instructions sequentially in memory unless a program instruction causes the PC to be otherwise altered. There are various instruc- tions that can modify the value of the PC; specifically, conditional branching instructions, direct jumps and calls, and returns from subroutines. Additionally, interrupts (when enabled) can cause the program flow to deviate from its otherwise sequential scheme.

6.2 Conditional Branching

The MSC1210 contains a suite of instructions that, as a group, are referred to as “conditional branching” instructions. These instructions cause the program execution to follow a non-sequential path if a certain condition is true.

Let us use the JB instruction as an example. This instruction means jump if bit set. An example of the JB instruction might be:

JB 45h,HELLO

NOP

HELLO:....

In this case, the MSC1210 will analyze the contents of bit 45H. If the bit is set, program execution will jump immediately to label HELLO, skipping the NOP instruction. If the bit is not set, the conditional branch fails and program execu- tion continues as usual with the NOP instruction that follows.

Conditional branching is really the fundamental building block of program logic because all decisions are accomplished by using conditional branching. Con- ditional branching can be thought of as the “IF ... THEN” structure of assembly language.

Note:

Your program may only branch to instructions located within 128 bytes prior to, or 127 bytes after the address that follows the conditional branch instruction. This means that in the above example, the label HELLO must be within −128 bytes to +127 bytes of the memory address that contains the conditional branching instruction.

6.3 Direct Jumps

While conditional branching is extremely important, it is often necessary to make a direct branch to a given memory location without basing it on a given logical decision. This is equivalent to saying GOTO in Basic. In this case, the program flow will continue at a given memory address without considering any conditions.

This is accomplished with the MSC1210 using direct jump and call instructions. As illustrated in the last paragraph, this suite of instructions causes program flow to change unconditionally.

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