3.3.1 Supervisor Mode

As will become clearer in later chapters, complex systems are often implemented as several programs that communicate with each other. These programs may run under the command of an operating system. It may be desirable to provide hardware checks to ensure that the programs do not interfere with each other—for example, by erroneously writing into a segment of memory used by another program. Software debugging is important but can leave some problems in a running system; hardware checks ensure an additional level of safety.

In such cases it is often useful to have a supervisor mode provided by the CPU. Normal programs run in user mode. The supervisor mode has privileges that user modes do not. For example, we will study memory management systems in 3.5 that allow the physical addresses of memory locations to be changed dynamically. Control of the memory management unit is typically reserved for supervisor mode to avoid the obvious problems that could occur when program bugs cause inadvertent changes in the memory management registers, effectively moving code and data in the middle of program execution.

Not all CPUs have supervisor modes. Many DSPs, including the C55x, do not provide one. The ARM, however, does have a supervisor mode. The ARM instruction that puts the CPU in supervisor mode is called SWI:

SWI CODE_1

It can, of course, be executed conditionally, as with any ARM instruction. SWI causes the CPU to go into supervisor mode and sets the PC to 0x08. The argument to SWI is a 24-bit immediate value that is passed on to the supervisor mode code; it allows the program to request various services from the supervisor mode.

In supervisor mode, the bottom five bits of the CPSR are all set to 1 to indicate that the CPU is in supervisor mode. The old value of the CPSR just before the SWI is stored in a register is called the saved program status register (SPSR). There are in fact several SPSRs for different modes; the supervisor mode SPSR is referred to as SPSR_svc.

To return from supervisor mode, the supervisor restores the PC from register r14 and restores the CPSR from the SPSR_svc.

3.3.2 Exceptions

An exception is an internally detected error. A simple example is division by zero. One way to handle this problem would be to check every divisor before division to be sure it is not zero, but this would both substantially increase the size of numerical programs and cost a great deal of CPU time evaluating the divisor’s value. The CPU can more efficiently check the divisor’s value during execution. Because the time at which a zero divisor will be found is not known in advance, this event is similar to an interrupt except that it is generated inside the CPU. The exception mechanism provides a way for the program to react to such unexpected events. Resets, undefined instructions, and illegal memory accesses are other typical examples of exceptions.

Just as interrupts can be seen as an extension of the subroutine mechanism, exceptions are generally implemented as a variation of an interrupt. Because both deal with changes in the flow of control of a program, it makes sense to use similar mechanisms. However, exceptions are generated internally.

Exceptions in general require both prioritization and vectoring. Exceptions must be prioritized because a single operation may generate more than one exception—for example, an illegal operand and an illegal memory access. The priority of exceptions is usually fixed by the CPU architecture. Vectoring provides a way for the user to specify the handler for the exception condition. The vector number for an exception is usually predefined by the architecture; it is used to index into a table of exception handlers.

3.3.3 Traps

A trap, also known as a software interrupt, is an instruction that explicitly generates an exception condition. The most common use of a trap is to enter supervisor mode. The entry into supervisor mode must be controlled to maintain security—if the interface between user and supervisor mode is improperly designed, a user program may be able to sneak code into the supervisor mode that could be executed to perform harmful operations.

The ARM provides the SWI interrupt for software interrupts. This instruction causes the CPU to enter supervisor mode. An opcode is embedded in the instruction that can be read by the handler.