HCS12 datasheet(430/452 Pages) FREESCALE

430 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

430

Freescale Semiconductor

C.9 Function Calls

Bank switching is a fairly common way of adapting a CPU with a 16-bit

address bus to accommodate more than 64 Kbytes of program memory

space. One of the most significant drawbacks of this technique has been the

requirement to mask (disable) interrupts while the bank page value was

being changed. Another problem is that the physical location of the bank

page register can change from one MCU derivative to another (or even due

to a change to mapping controls by a user program). In these situations, an

operating system program has to keep track of the physical location of the

page register. The CPU12 addresses both of these problems with the

uninterruptible CALL and return-from-call (RTC) instructions.

The CALL instruction is similar to a JSR instruction, except that the

programmer supplies a destination page value as part of the instruction.

When CALL executes, the old page value is saved on the stack and the new

page value is written to the bank page register. Since the CALL instruction

is uninterruptible, this eliminates the need to separately mask off interrupts

during the context switch.

The CPU12 has dedicated signal lines that allow the CPU to access the

bank page register without having to use an address in the normal 64-Kbyte

address space. This eliminates the need for the program to know where the

page register is physically located.

The RTC instruction is similar to the RTS instruction, except that RTC uses

the byte of information that was saved on the stack by the corresponding

CALL instruction to restore the bank page register to its old value. Although

a CALL/RTC pair can be used to access any function subroutine regardless

of the location of the called routine (on the current bank page or a different

page), it is most efficient to access some subroutines with JSR/RTS

instructions when the called subroutine is on the current page or in an area

of memory that is always visible in the 64-Kbyte map regardless of the bank

page selection.

Push and pull instructions can be used to stack some or all the CPU

registers during a function call. The CPU12 can push and pull any of the

CPU registers A, B, CCR, D, X, Y, or SP.

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