ARM Overview: Difference between revisions

For exceptions, ARM uses a table similar to the IVT of the real mode x86. The table consists of a number of 32-bit entries. Each entry is an instruction (ARM instructions are 4bytes4 bytes in length.) that jumps to the appropriate handler.

Take note of that, and understand the design impact it imposes: On x86, the hardware vector table holds the addresses of handler routines. On ARM, the hardware vector table holds actual instructions. These instructions must fit into 4 bytes. This is actually not a big deal since all ARM instructions (assuming ARM mode and not Thumb, or Jazelle) are actually 4Bytes4 bytes anyway. The general idea is to emulate the behaviour of something like the x86 and simply place a jump instruction into the actual vector table, so that upon indexing into the table, the ARM processor is made to act as if it jumped to an address contained in the jump instruction. From there, consistency is obtained, and portability is eased.