[ale] No RAM = No BIOS?

[Alan Hightower]

On Aug 16, 2012, at 12:32 PM, "mike at trausch.us" <mike at trausch.us> wrote:

> Memory-mapped IO also depends on the existence of memory, in that
> hardware MMIO uses the address bus, and the address bus can't be used
> initially without having at least a minimum amount of RAM to address;
> that said, I couldn't tell you what the bare minimum is, but it's far,
> far less than we are capable of installing in hardware today.  :-)

I don't follow what you are trying to illustrate.  "Memory-mapped I/O" historically and simply refers to decoding something other than main SDRAM using the (then legacy) ISA /RD & /WR strobes such as peripheral registers and buffers which are traditionally selected by /IORD and /IOWR.  Many peripherals placed their transfer buffers in 'high' memory where there was typically a SDRAM hole between 640K and 1M.  They did this to optimize block transfers on PC/XT class hardware using a single 'rep movsw' to transfer an entire buffer.  In modern context with respect to PCs, it simply means anything other than main RAM you access in the physical memory space (eg without using inp/outp instructions) such as PCI and AGP apertures.  On most non-Intel architectures, everything is 'memory mapped' as there is only one unified address space.

-Alan