Below is from ARMv7 Architecture doc.
An exception is described as asynchronous if either of the following applies: — the exception is not generated as a result of direct execution or attempted execution of the instruction stream — the return address presented to the exception handler is not guaranteed to indicate the instruction that caused the exception.
I really don't quite understand , especially for the first scene.
Anyone can show me an example?
Thanks~
Probably the most common example would be interrupts (IRQs and FIQs). Interrupts are events coming from outside the processor (*), and so asynchronous to the processor's execution.
Take something like a UART/serial port as an example. The UART could be configured to generate an interrupt when it receives a packet. When we receive an interrupt from the UART, it's due to the receipt of a packet - not directly the execution of an instruction on the processor. It's essentially random (**) when we get the interrupt, so the instruction pointed at by the LR tells us nothing about the interrupt.
(* processors can have interrupt output signals, for example to signal PMU overflow. However, these signals would be connected as inputs to an interrupt controller and it's the interrupt controller which would drive the interrupt signals into the processor.)
(** you could argue that for something like a periodic timer we the programmers have a fairly good idea when it will happen. But unless the processor was sitting in an idle loop or WFI, you couldn't predict accurately enough to know exactly instruction would be interrupted.)
On the "abort" side the most common cause is cache evictions from write-allocate caches getting a bus error back indicating that the physical address they were trying to write to didn't exist. This initial write into the cache succeeds (the VA to PA translation passes as there is a valid mapping in the MMU), the data is written to the cache, and the abort only happens "some time later" when that cache line is written back to memory.
Pete
Thanks very much , that's an excellent example ~
Thanks for your explanation.
So on the other side, exceptions directly caused by instructions like SMC, SVC are synchronous , right ?
Yes, SMC/HVC/SVC cause synchronous exceptions. The exception is directly connected to the execution of a specific instruction.
Understand, thanks a lot for your kind help~