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ST9 USER GUIDE

routine. Doing this preserves the contents of the whole group, and the reverse operation re-

stores them, as in the example:

; The main program uses the working register group 0

InterruptRoutine:

pushw

RPP

; Keep track of current group

srp

; Switch to group 1 (see text below for details)

...

; Body of the interrupt service routine

...

popw

RPP

; Restore whatever group was active

iret

; Return from interrupt

Supposing we could not switch working registers, we would have to push 16 bytes to the stack

to ensure that the contents of the working area have been preserved, and pop them back be-

fore returning. Obviously the example above is more efficient, both in code and data memory

size, and also in execution time.

You use register pointers to allocate the working registers in a particular group. When writing

in C or assembly language, you must position the working registers before you use them(2).

Switching groups involves the RPP register pair, made of the two registers RP0 and RP1.

These registers are directly accessible, but since their bits are laid out in a non-trivial manner,

it is recommended that you set them using only one of the srp, srp0 or srp1 instructions.

The registers are considered as sixteen groups of sixteen registers each. This is the way they

are represented in the register number summary table (See Table 1.). Use the numbers in this

table to refer to a register directly, e.g. when writing R35, this designates the fourth register of

group 2.

3.1.6.1 Switching the 16 Groups of Working Registers

This is done using the srp instruction. In spite of what we have explained up until now, and

how it is usually represented, the core does not actually divide the registers in 16 groups of 16

registers, but 32 blocks of 8 registers. This is why the srp instructions require arguments

ranging from 0 to 31 instead of 0 to 15. Here is how the Register Pointers select the desired

(2) Device Datasheet; Address spaces of the register file § 2.2.1 and following, system regis-

ters § 2.3.

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