HCS12 datasheet(78/452 Pages) FREESCALE

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S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

5.21 Jump and Subroutine Instructions

Jump (JMP) instructions cause immediate changes in sequence. The JMP

instruction loads the PC with an address in the 64-Kbyte memory map, and

program execution continues at that address. The address can be provided

as an absolute 16-bit address or determined by various forms of indexed

addressing.

Subroutine instructions optimize the process of transferring control to a

code segment that performs a particular task. A short branch (BSR), a jump

to subroutine (JSR), or an expanded-memory call (CALL) can be used to

initiate subroutines. There is no LBSR instruction, but a PC-relative JSR

performs the same function. A return address is stacked, then execution

begins at the subroutine address. Subroutines in the normal 64-Kbyte

address space are terminated with a return-from-subroutine (RTS)

instruction. RTS unstacks the return address so that execution resumes

with the instruction after BSR or JSR.

The call subroutine in expanded memory (CALL) instruction is intended for

use with expanded memory. CALL stacks the value in the PPAGE register

and the return address, then writes a new value to PPAGE to select the

memory page where the subroutine resides. The page value is an

immediate operand in all addressing modes except indexed indirect modes;

in these modes, an operand points to locations in memory where the new

page value and subroutine address are stored. The return from call (RTC)

instruction is used to terminate subroutines in expanded memory. RTC

unstacks the PPAGE value and the return address so that execution

resumes with the next instruction after CALL. For software compatibility,

CALL and RTC execute correctly on devices that do not have expanded

addressing capability. Table 5-21 summarizes the jump and subroutine

instructions.

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