The link register and program counter are key components of the ARM processor architecture. They play important roles in handling program execution flows and tracking instruction addresses. Understanding how these registers operate provides deeper insight into ARM’s unique architecture.
Link Register
The link register, abbreviated as LR, is a 32-bit processor register present in ARM CPUs. It contains the return address when a subroutine call instruction is executed. The LR register enables subroutine nesting and quick returns after subroutine execution.
When a BL (branch with link) instruction is executed, the address of the next instruction after the BL is loaded into the LR. Then the program counter is set to the branch target address to execute the subroutine code. Inside the subroutine, the LR holds the return address back to the main program flow.
To return from the subroutine, the MOV PC, LR instruction is used. This moves the LR return address into the program counter (PC), resuming execution at the instruction after the original BL call. Some other ARM instructions like BX LR can also be used to return from subroutines.
Without the LR, subroutine returns would need to store return addresses in memory. The dedicated LR register provides a faster and simpler mechanism for nested subroutine calls and returns. Compiler-generated code routinely relies on the LR for efficient function calling procedures.
Program Counter
The program counter (PC) is a 32-bit register that contains the address of the current instruction being executed by the processor. It is a key component of ARM’s instruction fetch pipeline.
On each clock cycle, the processor uses the PC to fetch the 4-byte instruction pointed to by the PC. After fetching, the PC increments by 4 to point to the next instruction in memory for the next cycle.
Some instructions also modify the PC to redirect execution. Branch instructions like B set the PC to a new branch target address. This changes the instruction fetch sequence to implement conditional program flow.
The PC starts at address 0x00000000 on processor reset. It increments through code, supporting sequential instruction execution. Branches let execution take redirects while subroutines use the LR to keep track of PC return points.
Interactions Between the Registers
The link register and program counter work closely together to handle program flow instructions like branches and subroutine calls/returns.
A BL instruction first stores the next PC value into the LR to set up the subroutine return address. The PC is then loaded with the branch target.
Inside the subroutine, reading the LR provides the return address. The MOV PC, LR instruction at the end of the subroutine restores the PC from the LR to resume original program flow.
In essence, the LR acts as a subroutine stack pointer while the PC directs ongoing sequential or branch instruction execution. Their coordinated actions enable robust program flow control.
Uses in Compiled Code
Modern compilers heavily utilize the link register and program counter when generating ARM machine code from high-level languages.
Subroutine call and return code is converted into BL and MOV PC instructions. The LR register is also preserved across subroutine calls according to ARM calling conventions.
Branch instructions like BEQ are generated from IF statements and other conditional constructs. The compiler inserts branches to implement program decision points.
Compiler optimization techniques also take advantage of the registers. For example, PC-relative addressing can avoid extra load instructions to access global data.
Understanding how compilers make use of the PC and LR can provide insight into the structure of generated code. This helps with debugging, reverse engineering, and program analysis.
Specialized Uses
Beyond their core functions, the link register and program counter enable some specialized uses:
- Position independent code relies on PC-relative addressing to work on any memory location.
- Some PC-relative data tables use the PC as a base address anchor for lookup logic.
- LR values stored on the stack provide stack trace backpoints for debugging call stacks.
- The LR can optionally be used as a general purpose register in Thumb code.
So while their main roles involve program flow, the versatile nature of the PC and LR registers lends itself to additional niche applications.
Exceptions and Low-Level Handling
The link register and program counter play key roles during low-level exception handling:
- The PC value when an exception occurs is saved to the LR of the exception handler context.
- The processor switches to a handler routine by updating the PC to point to a vector table entry.
- On return from exception, the PC is restored from the LR to resume normal flow.
- This mechanism passes context through the exception and back.
So beyond their uses in normal program operation, the PC and LR registers provide the backbone for robust exception processing.
Initialization and Configuration
For basic ARM operation, no special initialization is needed for the link register or program counter. Their standard roles are built into the hardware architecture.
However, some configuration options related to the registers include:
- PC alignment settings for fetch optimizations.
- Base address offsets for PC-relative instructions.
- Link register preservation conventions across routine calls.
- Stacking rules for nested routine calls.
Toolchains and operating systems may set up configurations to best match desired system behavior. Initialization code also often sets up stack pointers and initial register values.
Comparison to x86 Architecture
In the x86 architecture, the EIP (instruction pointer) register serves a similar role to the ARM program counter for instruction fetching. But x86 does not have a dedicated link register.
Instead, x86 uses the stack to store return addresses for call instructions. The x86 CALL saves the EIP value on the stack to be restored later by RET. This contrasts with ARM’s use of the dedicated LR register.
The x86 stack method can require more instruction overhead for call/return handling. The LR register approach on ARM provides optimization opportunities.
So the ARM link register is a key unique feature not found in other common architectures like x86. It contributes to ARM’s excellent subroutine efficiency.
Conclusion
The intricacies of the link register and program counter are central to understanding ARM’s execution model. They provide the foundation for branching, function calls, exception handling, and other core processor operations.
Mastering use of these registers is key to optimizing performance and writing efficient ARM assembler. Their capabilities enable clean high-level language mappings as well. Overall, the PC and LR registers are vital to unlocking the power of the ARM architecture.
