The program counter in ARM Cortex processors is stored in the PC (program counter) register. The PC register contains the address of the current instruction that is being executed by the processor. It is 32 bits wide and is updated during each fetch cycle to point to the next instruction.
Overview of Program Counter in ARM Cortex
The program counter (PC) plays a crucial role in the fetch-decode-execute cycle of ARM processors. Here is a brief overview of how it works:
- At the start of program execution, the PC is initialized to point to the first instruction in memory.
- During each clock cycle, the processor fetches the instruction pointed to by the PC.
- After fetching, the PC is incremented to point to the next instruction in sequence.
- The increment amount depends on the size of the current instruction. For a 32-bit Thumb instruction, the PC increments by 4 bytes.
- This cycle continues with the PC being incremented automatically by the processor to fetch sequential instructions.
- Control flow instructions like branches and function calls modify the PC to change the instruction fetch sequence.
- The PC thus maintains the current execution point and sequences the instruction fetch stream.
To summarize, the program counter PC holds the address of the next instruction to be executed. It drives the instruction fetch cycle and sequences program execution in ARM Cortex processors.
PC Register in ARM Cortex-M and Cortex-A Series
The PC register is implemented similarly across the ARM Cortex-M, Cortex-R and Cortex-A series of processor cores with minor variations:
ARM Cortex-M Series
- 32-bit PC register supporting both ARM and Thumb instruction sets.
- On reset, the PC is loaded from address 0x00000004.
- Writes to the PC are ignored in user/system mode.
- PC can be written in exception handlers through special register move instructions.
- PC supports unaligned address increments for efficient instruction fetching.
ARM Cortex-R Series
- 64-bit PC register supporting ARMv8-R architecture.
- On reset, PC is set to 0x00000000_00000000.
- PC can be read/written in both user and exception modes.
- PC updates only occur on explicit writes allowing precise exception handling.
ARM Cortex-A Series
- 64-bit PC register supporting ARMv8-A architecture.
- On reset, PC is set to 0x00000000_00008000.
- PC can be read/written in exception levels EL0/EL1/EL2/EL3.
- PC automatically increments during sequential instruction fetches.
So in summary, the PC register is a core component of the instruction fetch mechanism across all ARM Cortex processor families.
Reading and Writing the PC Register
There are some specific ways to read or write the PC register in ARM Cortex processors:
- In Cortex-M, PC can be read using the special register move instruction
- In Cortex-A/R, PC can be read like a general purpose register using
MRSin EL1/EL2/EL3 modes.
- Attempting to read PC directly in user mode results in an undefined instruction exception.
- In Cortex-M, writes to PC are ignored in user/system mode. Writes are allowed in exception handlers through
- In Cortex-A/R, PC can be written directly or through
MSRin EL1 and higher exception levels.
- Direct PC writes are possible in EL2/EL3 mode in Cortex-A for context switching.
So reading and writing the PC is privileged in Cortex-M, while Cortex-A/R provide more flexibility at higher exception levels.
How PC is Updated
There are several ways in which the program counter gets updated during execution in ARM Cortex processors:
- Sequential Increment – The PC is incremented after fetching every instruction to point to the next instruction. Increment amounts vary based on instruction size.
- Branch Instructions – Branches like B/BL modify the PC to point to a new target location for program flow change.
- Exceptions – On taking exceptions, the PC is saved to LR and exception handler address is loaded.
- Function Calls – BL/BLX instructions save PC to LR and load target address into PC.
- Direct Writes – PC can be directly modified through MSR instruction at higher privilege levels.
The common cases are sequential increment and updates through branches/function calls. Direct PC writes are less frequent but used for context switches.
Typical Uses of the PC Register
Here are some typical use cases of the program counter register in ARM Cortex processors:
- Instruction Fetch Sequencing – Updating PC each cycle drives the instruction fetch sequence.
- Branches – B/BL instructions target PC for code branching.
- Link Register – PC value is saved to the link register (LR) for function calls.
- Exception Handling – PC is saved to LR when exceptions occur.
- Debugging – Reading PC allows determining current instruction location.
- Context Switching – Writing PC enables process switch in OS context switches.
The key uses are instruction sequencing, branches, and linking related to function calls. Context switching and debugging related uses are less common.
Typical Problems Associated with PC
Some typical problems encountered when working with the program counter register are:
- Incorrect Branches – Errors in branch target address calculation lead to wrong PC updates.
- Stack Overflows – PC overflow due to lack of stack pointer checking in functions.
- Undefined Exceptions – Attempting to directly read/write PC in user mode causes exceptions.
- Lost Context – Forgetting to save/restore PC during context switches.
- Endless Loops – Accidental infinite loops if PC is corrupted.
Care must be taken when updating PC since incorrect values lead to wrong flow of control. Other issues like stack overflows, exceptions, and context switching errors must also be avoided.
Importance of Properly Managing the PC
Managing the program counter correctly is critical for several reasons:
- It determines the instruction execution sequence and program flow.
- Wrong PC values can cause code execution to go out of control.
- PC is used to store return addresses for function calls.
- PC needs to be properly saved/restored during context switches.
- PC is required for debugging and determining code location.
As the instruction fetch is driven by PC, it is essential to manage it correctly through branches, function calls and exception handling. Failure to do so results in catastrophic outcomes like crashes, hangs or memory corruptions.
Tools for Viewing and Managing PC
There are some commonly used tools for viewing and managing PC register contents:
- GDB – The GNU debugger allows viewing PC value and setting breakpoints.
- Inline Assembly – Embedded assembly allows direct PC access in C programs.
- JTAG Debugger – On-chip debug module provides debug exception and PC access.
- Perf Counters – Performance monitoring counters can profile PC and exceptions.
- Hypervisors – Hypervisor assisted debug exceptions and PC access.
GDB, JTAG, and inline assembly are common across software, OS, and firmware. Perf counters and hypervisors are used when debugging ADAS and other embedded systems.
- The program counter PC register holds address of next instruction to execute.
- PC drives the instruction fetch cycle and sequences execution.
- Branches, function calls, exceptions update PC to alter flow.
- PC needs to be managed carefully to avoid crashes.
- Tools like debuggers and profilers help view and analyze PC.
In summary, the program counter PC plays a central role in ARM Cortex processors. Managing it correctly is essential for proper code execution flow and avoiding problems. Familiarity with PC usage in branches, function calls, exceptions, and debugging is important for both software and firmware developers working with ARM Cortex cores.
Frequently Asked Questions
Q: Is the PC register present in all ARM Cortex cores?
Yes, the PC register is an integral part of the instruction fetch mechanism and is present in all variants of ARM Cortex processor cores including Cortex-M, Cortex-R and Cortex-A series.
Q: Does PC overflow on reaching max value?
In 32-bit Cortex-M cores, once PC reaches 0xFFFFFFFF it rolls over back to 0x00000000. In 64-bit Cortex-A and Cortex-R cores, PC is unlikely to overflow in practice.
Q: Can PC be accessed directly in user mode programs?
No, attempting to directly access PC in user mode results in an undefined instruction exception. PC access is allowed in exception handler modes based on privilege levels.
Q: How is PC value shown in ARM Cortex debuggers?
Debuggers like GDB display the PC value in hex along with the function name and offset corresponding to the PC address location.
Q: What happens when PC is corrupted?
If PC gets an unexpected corrupted value, then the processor will try to fetch instructions from wrong memory areas leading to crashes or hangs. Recovery may require hardware reset.
The program counter PC register is at the heart of instruction sequencing in ARM Cortex processors. It needs careful management via branches, link registers, exceptions handlers and context switching to maintain correct program execution flow. Familiarity with PC behavior is crucial for system, OS and firmware developers working on ARM Cortex platforms to avoid subtle and hard-to-debug issues.