Registers
Registers are a key element of AVR microcontrollers. There are various types of registers, including general-purpose, special-purpose, and status registers. General-purpose registers are used to store temporary data. Special registers control microcontroller functions, such as timers or ADC. Status registers store information about the processor state, such as carry or zero flags. Each register has a specific function and is accessible through particular assembler instructions. Registers allow quick access to data and control over the processor.
AVR CPU General Purpose Working Registers (R0-R31)
R0-R15: Basic general-purpose registers are used for various arithmetic and logical operations.
R16-R31: General-purpose registers that can be used with immediate
Comparison Table: AVR Registers R0–R15 vs. R16–R31
| Feature | R0–R15 | R16–R31 |
|---|---|---|
| Instruction compatibility | Cannot be used with immediate instructions (LDI, CPI, ORI, ANDI) | Fully compatible with immediate instructions |
| Opcode encoding | Not encodable in 4‑bit register fields | Encodable in all immediate‑type formats |
| Typical compiler usage | Less preferred; used for temporary or special roles | Preferred for arithmetic and logic operations |
| Special-purpose roles | R0: scratch<br>R1: constant zero | R26–R31 used as X/Y/Z pointer registers |
| Pointer support | No | Yes: X, Y, Z pointers |
| Efficiency | Less efficient for immediate operations | More efficient due to shorter opcodes |
| Multiplication involvement | R0/R1 used implicitly for results | Not used by multipliers |
| Suitability in ASM | Limited; avoid with immediates; R0/R1 have special rules | Recommended for general ALU operations |
| GCC policy | R1 must remain 0; R0 is volatile | Treated as normal general-purpose registers |
The X-register, Y-register, and Z-register: address pointers for indirect addressing R26-R31: These registers serve as 16-bit address pointers for indirect addressing of the data space. They are defined as X, Y, and Z registers.
Other registers:
RAMPX, RAMPY, RAMPZ: Registers concatenated with the X-, Y-, and Z-registers, enabling indirect addressing of the entire data space on MCUs with more than 64 KB of data space, and constant data fetch on MCUs with more than 64 KB of program space.
RAMPD: Register concatenated with the Z-register, enabling direct addressing of the whole data space on MCUs with more than 64 KB data space.
EIND: Register concatenated with the Z-register, enabling indirect jump and call to the entire program space on MCUs with more than 64K words (128 KB) of program space.
Table 1: Register properties
| Register | Purpose | Combined With | Enables |
|---|---|---|---|
| RAMPX | Extends data addressing | X register | Access to data above 64 KB using X |
| RAMPY | Extends data addressing | Y register | Access to data above 64 KB using Y |
| RAMPZ | Extends data and program addressing | Z register | Access to data and constants above 64 KB using Z |
| RAMPD | Extends data addressing | Z register | Direct access to data above 64 KB |
| EIND | Extends program addressing | Z register | Jumps and calls to program memory above 128 KB |
All of the registers RAMPX, RAMPY, RAMPZ, RAMPD, and EIND are located in the extended I/O space of AVR microcontrollers. This area typically occupies addresses 0x3B–0x3F, although the exact address of each register depends on the specific AVR device. The precise locations can always be found in the Register Summary section of the device datasheet.
