Significance of Reset Vector in Modern Processors

Question

I am trying to understand how computer boots up in very detail.

I came across two things which made me more curious,
1. RAM is placed at the bottom of ROM, to avoid Memory Holes as in Z80 processor.
2. Reset Vector is used, which takes the processor to a memory location in ROM, whose contents point to the actual location (again ROM) from where processor would actually start executing instructions (POST instruction). Why so?

If you still can't understand me, this link will explain you briefly, http://lateblt.tripod.com/bit68.txt

Answer 1

The processor logic is generally rigid and fixed, thus the term hardware. Software is something that can be changed, molded, etc. thus the term software.

The hardware needs to start some how, two basic methods,

1) an address, hardcoded in the logic, in the processors memory space is read and that value is an address to start executing code

2) an address, hardcoded in the logic, is where the processor starts executing code

When the processor itself is integrated with other hardware, anything can be mapped into any address space. You can put ram at address 0x1000 or 0x40000000 or both. You can map a peripheral to 0x1000 or 0x4000 or 0xF0000000 or all of the above. It is the choice of the system designers or a combination of the teams of engineers where things will go. One important factor is how the system will boot once reset is relesed. The booting of the processor is well known due to its architecture. The designers often choose two paths:

1) put a rom in the memory space that contains the reset vector or the entry point depending on the boot method of the processor (no matter what architecture there is a first address or first block of addresses that are read and their contents drive the booting of the processor). The software places code or a vector table or both in this rom so that the processor will boot and run.

2) put ram in the memory space, in such a way that some host can download a program into that ram, then release reset on the processor. The processor then follows its hardcoded boot procedure and the software is executed.

The first one is most common, the second is found in some peripherals, mice and network cards and things like that (Some of the firmware in /usr/lib/firmware/ is used for this for example).

The bottom line though is that the processor is usually designed with one boot method, a fixed method, so that all software written for that processor can conform to that one method and not have to keep changing. Also, the processor when designed doesnt know its target application so it needs a generic solution. The target application often defines the memory map, what is where in the processors memory space, and one of the tasks in that assignment is how that product will boot. From there the software is compiled and placed such that it conforms to the processors rules and the products hardware rules.

Answer 2

It completely varies by architecture. There are a few reasons why cores might want to do this though. Embedded cores (think along the lines of ARM and Microblaze) tend to be used within system-on-chip machines with a single address space. Such architectures can have multiple memories all over the place and tend to only dictate that the bottom area of memory (i.e. 0x00) contains the interrupt vectors. Then then allows the programmer to easily specify where to boot from. On Microblaze, you can attach memory wherever the hell you like in XPS.

In addition, it can be used to easily support bootloaders. These are typically used as a small program to do a bit of initialization, then fetch a larger program from a medium that can't be accessed simply (e.g. USB or Ethernet). In these cases, the bootloader typically copies itself to high memory, fetches below it and then jumps there. The reset vector simply allows the programmer to bypass the first step.