General purpose register file, X-register, y-register, and z- register – Rainbow Electronics AT90S2313 User Manual
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AT90S2313
0839I–AVR–06/02
General Purpose
Register File
Figure 6 shows the structure of the 32 general purpose registers in the CPU.
Figure 6. AVR CPU General Purpose Working Registers
All the register operating instructions in the instruction set have direct and single-cycle
access to all registers. The only exception is the five constant arithmetic and logic
instructions SBCI, SUBI, CPI, ANDI, ORI between a constant and a register and the LDI
instruction for load immediate constant data. These instructions apply to the second half
of the registers in the Register File (R16..R31). The general SBC, SUB, CP, AND, OR,
and all other operations between two registers or on a single register apply to the entire
Register File.
As shown in Figure 6, each register is also assigned a data memory address, mapping
them directly into the first 32 locations of the user Data Space. Although the Register
File is not physically implemented as SRAM locations, this memory organization pro-
vides great flexibility in access of the registers, as the X-, Y-, and Z-registers can be set
to index any register in the file.
X-register, Y-register, and Z-
register
The registers R26..R31 have some added functions to their general purpose usage.
These registers are the address pointers for indirect addressing of the Data Space. The
three indirect address registers X, Y and Z are defined in Figure 7.
Figure 7. X-, Y-, and Z-Registers
7
0
Addr.
R0
$00
R1
$01
R2
$02
…
R13
$0D
General
R14
$0E
Purpose
R15
$0F
Working
R16
$10
Registers
R17
$11
…
R26
$1A
X-register Low Byte
R27
$1B
X-register High Byte
R28
$1C
Y-register Low Byte
R29
$1D
Y-register High Byte
R30
$1E
Z-register Low Byte
R31
$1F
Z-register High Byte
15
0
X-register
7
0
7
0
R27 ($1B)
R26 ($1A)
15
0
Y-register
7
0
7
0
R29 ($1D)
R28 ($1C)
15
0
Z-register
7
0
7
0
R31 ($1F)
R30 ($1E)