You mean with parallel address and data bus? That would be a whole lot of pins and quite a large effort to implement, not to speak of getting the timing right,

You mean with parallel address and data bus? That would be a whole lot of pins and quite a large effort to implement, not to speak of getting the timing right, when going beyond a clock rate of 10MHz or so. In the MCU world (quad) SPI RAM is favored, if external RAM is needed. That can be done with the RP2040 (well, you'd have to move the content yourself, there is no hardware support to transparently access external RAM, like in some other MCUs). But what exactly do you have in mind with the 'instant booting computer experience'? The RP2040 has 264KiB of RAM already (more than the address space of a PDP-11), so surely that qualifies as 'computer'. That should be enough to even emulate a 8 bit Home Computer (well, it has been done for 6502 and Z80 CPUs and whole CP/M machines and ZX Spectrum). Not that I get this obsession with 'instant on'. My chromebook is ready for me the moment (well, within a blink of an eye) I open the lid. My desktop needs a minute or so, but I reboot it only when there's a kernel update ...