The address space for a 32 bit system is 0x00000000 to 0xffffffff. From what I understand, this address space will be split among the system memory (RAM), ROM and memory-mapped peripherals. If the entire address space were used to address on the 4GB RAM, all RAM bytes would be accessible. But the address space being distributed with other memory mapped peripherals, does this mean that some RAM will be unaddressable/unutilized?
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Here is the memory map of a typical x86 system. As you can see, the lower ranges of memory are riddled with BIOS and ROM data with small gaps in between. There's a substantial portion reserved for memory mapped devices in the upper ranges. All of these details may vary between platforms. It's nothing short of a nightmare to detect which memory areas that can be safely used.
The kernel also typically reserves a large portion of the available memory for its internals, buffers and cache.
With the advent of virtual addressing, the kernel can advertise the address space as one consistent and gapless memory range, while that is not necessarily true behind the scenes.
