NEC V850E/PH2, User Manual

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Chapter 3 CPU Functions
User’s Manual U16580EE3V1UD00
3.2.1 Program register set
The program register set includes general-purpose registers and a program counter.
(1) General-purpose registers (r0 to r31)
Thirty-two general-purpose registers, r0 to r31, are available. All of these registers can be used as
a data variable or address variable.
However, r0 and r30 are implicitly used by instructions and care must be exercised when using
these registers. r0 always holds 0 and is used for operations that use 0 or offset 0 addressing. r30
is used as a base pointer when performing memory access with the SLD and SST short
instructions.
Also, r1, r3 to r5, and r31 are implicitly used by the assembler and C compiler. Therefore, before
using these registers, their contents must be saved so that they are not lost, and they must be
restored to the registers after use. There are cases when r2 is used by the real-time OS. If r2 is not
used by the real-time OS, r2 can be used as a variable register.
(2) Program counter (PC)
This register holds the address of the instruction under execution. The lower 26 bits of this register
are valid, and bits 31 to 26 are fixed to 0. If a carry occurs from bit 25 to bit 26, it is ignored.
Bit 0 is fixed to 0, and branching to an odd address cannot be performed.
Figure 3-2: Program Counter (PC)
Table 3-1: Program Registers
Name Usage Operation
r0 Zero register Always holds 0
r1 Assembler-reserved register Working register for generating 32-bit immediate
r2 Address/data variable register (when r2 is not used by the real-time OS to be used)
r3 Stack pointer Used to generate stack frame when function is called
r4 Global pointer Used to access global variable in data area
r5 Text pointer Register to indicate the start of the text area (area for placing program
code)
r6 to r29 Address/data variable register
r30 Element pointer Base pointer when memory is accessed
r31 Link pointer Used by compiler when calling function
31 26 25 1 0
After reset
00000000H
PC Fixed to 0 Instruction address under execution 0