Raspberry Pi Zero W Project Part 3 - Software Interrupt & Handler example

For ARM architecture, the instruction for software interrupt is SWI or SVC (ARMv7).

Using the instruction is very simple:

SVC #imm

When an ARM processor executes the instruction, it will switch to SVC/SWI mode and jump to the SWI or SVC hander in exception vector table specified by Vector Base Address Register (VBAR).

Similar to previous parts, please get from repo https://github.com/champyen/rpiz_bare_metal.git 

please checkout the commit: 637086c2

$ git checkout 637086c2

It is not difficult to use SWI/SVC instruction. But it requires experience to apply it well. Using SWI/SVC instrution for system calls, we need to:

• design system call table ( SWI number , system call pair)
  
• implement SWI/SVC handler, get the system call number
  
• implement system calls with corresponding number
  

For get the SWI (or system call) number, it is describe in ARM's "SWI Handler's" document. The SWI number is encoded as lowest 24 bit in SWI/SVC instruction. Therefore we could get SWI/SVC number by fetching the instruction itself and clearing the MSb 8bits with BIC instruction right after entering SWI hander:

ldr     r0, [lr,#-4]
bic     r0, r0, #0xff000000

After getting SWI number, we could pass it to system_handler as an argument saved in R0. The system_hander is very similar to ISR, but we don't need to check the hardware status to know which hardware interrupts CPU. We just need to handler the SWI or system call by the SWI number. Of course, as IRQ has Interrupt latency SWI/SVC instruction consumes more cycles than normal instructions for most CPUs.

For Application processor it is easy to understand the meaning of mode switching and SWI handling. In fact, Cortex-R, and Cortex-M processor also has SWI/SVC instruction. For most Embedded / RTOS developers, they think it is useless or trivial in such processor.

For some embedded or RTOS, mutual-exclusive applications can be loaded on demand. This can be done by well-designed linker-script (e.g.: different LAs with same). For such Embedded / RTOS, SWI / SVC instruction is useful to maintain an API layer between Applications and Kernel. An intuitive / naive way is to setup and maintain table of function pointers on the kernel side (Of course we still need to setup API index for using the function as SWI number). For development it has an drawback: it is hard to maintain or adjust or expand the table. With SWI/SVC instruction, no table forwarding is needed. Besides it provides flexibility to group system calls and reserve numbers for future needs. (Since for table-based method, it required to reserve a huge table).