SC9RS08KA2 datasheet(62/134 Pages) FREESCALE

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Chapter 8 Central Processor Unit (RS08CPUV1)

SC9RS08KA2 Series Data Sheet, Rev. 1

Freescale Semiconductor

expression in the operand field of the branch instruction; the assembler calculates the difference between

the location counter (which points at the next address after the branch instruction at the time) and the

address represented by the label or expression in the operand field. This difference is called the offset and

is an 8-bit two’s complement number. The assembler stores this offset in the object code for the branch

instruction.

During execution, the CPU evaluates the condition that controls the branch. If the branch condition is true,

the CPU sign-extends the offset to a 14-bit value, adds the offset to the current PC, and uses this as the

address where it will fetch the next instruction and continue execution rather than continuing execution

with the next instruction after the branch. Because the offset is an 8-bit two’s complement value, the

destination must be within the range –128 to +127 locations from the address that follows the last byte of

object code for the branch instruction.

A common method to create a simple infinite loop is to use a branch instruction that branches to itself. This

is sometimes used to end short code segments during debug. Typically, to get out of this infinite loop, use

the debug host (through background commands) to stop the program, examine registers and memory, or

to start execution from a new location. This construct is not used in normal application programs except

in the case where the program has detected an error and wants to force the COP watchdog timer to timeout.

(The branch in the infinite loop executes repeatedly until the watchdog timer eventually causes a reset.)

8.3.3

Immediate Addressing Mode (IMM)

In this addressing mode, the operand is located immediately after the opcode in the instruction stream. This

addressing mode is used when the programmer wants to use an explicit value that is known at the time the

program is written. A # (pound) symbol is used to tell the assembler to use the operand as a data value

rather than an address where the desired value will be accessed.

The size of the immediate operand is always 8 bits. The assembler automatically will truncate or extend

the operand as needed to match the size needed for the instruction. Most assemblers generate a warning if

a 16-bit operand is provided.

It is the programmer’s responsibility to use the # symbol to tell the assembler when immediate addressing

will be used. The assembler does not consider it an error to leave off the # symbol because the resulting

statement is still a valid instruction (although it may mean something different than the programmer

intended).

8.3.4

Tiny Addressing Mode (TNY)

TNY addressing mode is capable of addressing only the first 16 bytes in the address map, from $0000 to

$000F. This addressing mode is available for INC, DEC, ADD, and SUB instructions. A system can be

optimized by placing the most computation-intensive data in this area of memory.

Because the 4-bit address is embedded in the opcode, only the least significant four bits of the address must

be included in the instruction; this saves program space and execution time. During execution, the CPU

adds 10 high-order 0s to the 4-bit operand address and uses the combined 14-bit address ($000x) to access

the intended operand.

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