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3.1 Register Partitioning

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In order to reduce the number of register saves required for typical procedure calls, the PA-RISC general and floating-point register files have been divided into partitions designated as callee-saves and caller-saves. The names of these partitions indicate which procedure takes responsibility for preserving the contents of the register when a call is made.

If a procedure uses a register in the callee-saves partition, it must save the contents of that register immediately after procedure entry and restore the contents before the exit. Thus, the contents of all callee-saves registers are guaranteed to be preserved across procedure calls.

A procedure is free to use the caller-saves registers without saving their contents on entry. However, the contents of the caller-saves registers are not guaranteed to be preserved across calls. If a procedure has placed a needed value in a caller-saves register, it must be stored to memory or copied to a callee-saves register before making a call.

These partitions and the other dedicated registers are shown in Figures 3-1 and 3-2.

Figure 3-1 Register Partitioning

[Figure 3-1]

Figure 3-2 Floating-Point Registers

[Figure 3-2]



Chapter 3 Internet and Interoperability

3.2 Other Register Conventions