The ARM Cortex-M0 is the smallest and simplest processor in the Cortex-M series of ARM processors. It is designed for microcontroller applications that require minimal power consumption and cost. The Cortex-M0 has a 32-bit RISC architecture with a reduced instruction set optimized for embedded applications. One of the key features of any processor is the number of registers it provides, as registers store temporary data during program execution and significantly impact performance.
Registers in Cortex-M0
The ARM Cortex-M0 contains a total of 16 general purpose 32-bit registers, designated as R0-R15. These registers can be used for any purpose by the programmer. In addition, there is one 32-bit Program Counter (PC) register which contains the address of the current instruction being executed. The Cortex-M0 also contains a 32-bit Program Status Register (PSR) which holds status and control bits such as interrupt enable/disable, operating mode, etc.
Therefore, to summarize:
- 16 x 32-bit general purpose registers (R0-R15)
- 1 x 32-bit Program Counter (PC) register
- 1 x 32-bit Program Status Register (PSR)
That’s a total of 18 core registers in the Cortex-M0 processor. Let’s look at the role of each these registers in more detail:
General Purpose Registers
The 16 general purpose registers, R0-R15, can be used to store operands and results during program execution. They can be accessed very quickly by instructions, making them ideal for holding intermediate values locally within a function.
Some key notes on the general purpose registers:
- R0-R12 are available for general use by programmer
- R13 is designated as the Stack Pointer (SP)
- R14 is designated as the Link Register (LR)
- R15 is designated as the Program Counter (PC)
While R13-R15 have special designations, they can also be used as general purpose registers if needed. The stack pointer and link register are useful for stack operations and subroutine linking respectively.
Program Counter Register
The Program Counter (PC) register contains the address of the next instruction to be fetched and executed by the processor. It is automatically incremented during each instruction cycle. Branch and jump instructions allow the PC to be modified to implement changes in program flow.
Some key notes on the Program Counter register:
- 32-bit wide to support large memory space
- On reset, PC is loaded with start address of the program
- Automatically incremented each clock cycle
- Modified by branch and jump instructions
Program Status Register
The Program Status Register (PSR) contains important status and control bits related to the processor operation. These include:
- Interrupt enable/disable bits
- Processor mode selection
- Overflow and carry flags
- Zero and negative status flags
The PSR allows interrupts to be enabled/disabled globally. It also controls the processor mode – Thread vs Handler mode. Setting the overflow, carry, zero and negative status flags allows efficient conditional testing of arithmetic operation results.
Accessing the Registers
The general purpose registers R0-R12 are freely available to use as operands in ARM instructions. To load a value into a register, the LDR instruction is used. For example: LDR R1, =0x2FF // Load hex value 0x2FF into R1
The value in a register can be stored to memory using the STR instruction: STR R1, [R2] // Store R1 value into memory at address in R2
Arithmetic and logical instructions can operate directly on registers: ADD R3, R1, R2 // R3 = R1 + R2 AND R4, R1, R2 // Bitwise AND between R1 and R2
The stack pointer R13 and link register R14 are accessed just like general purpose registers using LDR, STR and other instructions. The program counter R15 can be read using LDR instructions, but writing to it directly is not allowed. R15 must be modified using branch instructions.
The program status register can be read and written to using special instructions such as MRS, MSR, CPS. For example: MRS R1, PSR // Copy PSR into R1 MSR PSR, R2 // Copy R2 into PSR CPSID I // Disable interrupts CPSIE I // Enable interrupts
Locating Register Definitions
The official ARM documentation contains detailed descriptions of all the registers in the Cortex-M0 including bit assignments. Some key documents are:
- Cortex-M0 Devices Generic User Guide
- Cortex-M0 Technical Reference Manual
- ARMv6-M Architecture Reference Manual
These documents provide programmers with insight into the internal workings of the registers. They describe options for configuring the PSR mode bits, interrupt enables, stack pointer, etc. Register diagrams show the bit positions which is useful when directly manipulating the registers through code.
In addition, the header files provided with compiler toolchains will contain definitions for the registers and status bits. Referring to these headers when programming will ensure the correct registers and bit patterns are used.
Importance of Registers
Registers play a vital role in any processor architecture. Having a larger number of general purpose registers allows more intermediate values to be stored close to the CPU, avoiding slow external memory access. The Cortex-M0 provides 16 registers which is sufficient for simple embedded applications.
Special purpose registers like the program counter and program status register control critical aspects of the processor operation. Setting the PSR configuration correctly is essential for proper functioning of the interrupts and operating modes.
Understanding how to efficiently use the available registers is key to writing high performance Cortex-M0 code. Registers provide quick access to operands and results, significantly speeding up execution compared to using only memory variables.
Conclusion
The ARM Cortex-M0 contains a total of 18 registers including 16 general purpose registers, a program counter, and a program status register. These provide quick access to data during program execution. Proper utilization of the available registers can lead to increased performance compared to relying solely on memory access. Consult the ARM documentation and header files for detailed information on the register definitions and bit assignments.
