The ARM RISC architecture

The ARM RISC architecture

It is probably fair to say that the ARM RISC architecture is really what RISC is all about. Small simple processors that provide adequate performance for their intended marketplace. For ARM, this was not the area of blinding performance but in the then embryonic mobile and handheld world where power consump-tion is as important as anything else. ARM also brought in the concept of the fabless semiconductor company where they licence their designs to others to build. As a result, if you want an ARM processor then you need to go to one of the 50+ licenced manufac-turers. As a result, ARM processor architectures power the bulk of the digital mobile phones and organisers available today.

The ARM register set

The architecture uses standard RISC architecture techniques (load-store architecture, simple addressing modes based on regis-ter contents and instruction information only, fixed length in-structions etc.,) and has a large 32 register file which is banked to provide a programming model of 16 registers with additional registers from the 32 used when the processor handles exceptions. This is called register banking where some of the spare registers are allocated as replacements for a selected set of the first 16 registers. This means that there is little need to save the registers during a context switch. This mechanism is very similar to register windowing in reality.

Two registers have special usage: register 14 is used as a link register and holds the address of the next instruction after a branch and link instruction. This permits the software flow to return using this link address after a subroutine has been executed. While it can be used as a general purpose register, care has to be taken that its contents are not destroyed so that when a return is executed, the program returns to the wrong address or even one that has no associated memory! Register 15 is used as the program counter. The ARM architecture uses a fixed 4 byte instruction word and supports the aligned instruction organisation only. This means that each instruction must start on a word boundary and also means that the lowest two bits in the program counter are always set to zero. In effect, this reduces the register to only 30 bits is size. Looking at the PC can be a little strange as it points not to the currently executing instruction but to two instructions after that. Useful to remember when debugging code.

While registers 14 and 15 are already allocated to special use, the remaining 14 registers may also be used for special functions. These definitions are not forced in hardware as is the case with the previous two examples, but are often enforced in software either through the use of a programming convention or by a compiler. Register 13 is frequently used as a stack pointer by convention but other registers could be used to fulfil this function or to provide additional stack pointers.

Exceptions

Exception processing with the ARM architecture is a little more complicated in that it supports several different exception processing modes and while it could be argued that these are nothing more than a user mode and several variants of a supervi-sor mode (like many other RISC architectures), they are suffi-ciently different to warrant their separate status.

The processor normally operates in the user mode where it will execute the bulk of any code. This gives access to the 16 register program file as previously described. To get into an exception mode from a user mode, there are only five methods to do so. The common methods such as an interrupt, a software interrupt, memory abort and the execution of an undefined in-struction are all there. However a fifth is supported which is designed to reduce the latency time taken to process a fast inter-rupt. In all cases, the processor uses the register banking to preserve context before switching modes. Registers 13 and 14 are both automatically banked to preserve their contents so that they do not need to be saved. Once in the exception handler, register 14 is used to hold the return address ready for when the handler completes and returns and register 13 provides a unique stack pointer for the handler to use. Each exception will cause the current instruction to complete and then the execution flow will change to the address stored in the associated location in the vector table. This technique is similar to that used with both CISC and RISC processors.

If the handler needs to use any of the other registers, they must be saved before use and then restored before returning. To speed this process up, there is a fifth mode called the fast interrupt mode where registers 8 to 12 are also banked and these can be used by the handler without the overhead of saving and restoring. This is known as the fast interrupt mode.

The exception modes do not stop there. There is also a sixth mode known as the system mode which is effectively an enhanced user mode in that it uses the user mode registers but is provided with privileged access to memory and any coprocessors that might be present.

The Thumb instructions

The ARM processor architecture typically uses 32 bit wide instructions. Now bearing in mind it is targeted at portable appli-cations where power consumption is critical, the combination of RISC architectures coupled with 32 bit wide instructions leads to a characteristic known as code expansion. RISC works by simpli-fying operations into several simple instructions. Each instruction is 32 bits in size and a typical sequence may take three instructions. This means that 12 bytes of program storage are needed to store the instructions. Compare this to a CISC architecture that could do the same work with one instruction with 6 bytes. This means that the RISC solution requires twice as much program storage which means twice the memory cost and power consumption (this is a bit of a simplification but the increase is very significant). And yes, this argument is very similar to that put forward when the first microprocessors appeared where memory was very expensive and it was advantageous to uses as little of it as possible — hence the CISC architectures.

ARM's solution to this was to add a new set of instructions to the instructions set called the Thumb instructions. These are reduced in functionality but are only 16 bits in size and therefore take less space. As the processor always brings in data in 32 bit words, two Thumb instructions are brought in and executed in turn. Thumb instructions are always executed in a special Thumb mode which is controlled by a Thumb bit in the status register. This requires some software support so that the compilers can insure that the Thumb instruction sequences are only executed by the CPU when it is in its Thumb mode, but the benefit is a greatly reduced code size, approaching that offered by CISC processors.