1. Absolute addresses¶

A lot of the assembly code currently runs in real mode, which meansabsolute addresses are used instead of virtual addresses as in therest of the kernel. To translate an absolute address to a virtualaddress you can lookup in System.map, add __PAGE_OFFSET (0x10000000currently).

2. HPMCs¶

When real-mode code tries to access non-existent memory, you’ll getan HPMC instead of a kernel oops. To debug an HPMC, try to findthe System Responder/Requestor addresses. The System Requestoraddress should match (one of the) processor HPAs (high addresses inthe I/O range); the System Responder address is the address real-modecode tried to access.

Typical values for the System Responder address are addresses largerthan __PAGE_OFFSET (0x10000000) which mean a virtual address didn’tget translated to a physical address before real-mode code tried toaccess it.

3. Q bit fun¶

Certain, very critical code has to clear the Q bit in the PSW. Whathappens when the Q bit is cleared is the CPU does not update theregisters interruption handlers read to find out where the machinewas interrupted - so if you get an interruption between the instructionthat clears the Q bit and the RFI that sets it again you don’t knowwhere exactly it happened. If you’re lucky the IAOQ will point to theinstruction that cleared the Q bit, if you’re not it points anywhereat all. Usually Q bit problems will show themselves in unexplainablesystem hangs or running off the end of physical memory.