How can we say how much ROM is supported by a Uc?
For example,for 8051,what is the maximum ROM that it can supports?
Thanks in advance...
As noted above, "RAM" and "ROM" are irrelevant - all the CPU knows is Address Space.
Of course, further study of the Datasheet will be necessary in order for the designer to populate that address space with actual memory devices - RAM or ROM or whatever...
Neil & Per thanks for the discussion..
Philips 8051 MX has Unified 64 MB Program Memory,how is it possible?
I haven't looked at the chip but there are always possible to build chips with huge address ranges.
One way is to have banked/indexed memory, where you write to SFR to select active bank.
And if using any hole in the instruction map, you can add extra instructions that makes absolute jumps or calls directly to other banks.
And it's possible to design a chip that either requires all ISR to start in a fixed bank, or where each ISR have a special SFR to specify what bank the code is in.
The possibilities are huge - but unless the compiler knows about these additional features, or have support for calling customiseable helper functions, such chips can be quite hard to use.
Everyone "knows" that the 32-bit Intel 80386 only supports 4GB memory, because of the 32-bit address and data busses. But it also have a couple of other pins telling what mode it is in, so it could just as well be used with 4GB of user addresses and 4GB of system addresses - and potentially 4GB of monitor addresses. And when the successors got PAE (physical address extension), they started to support 64GB.
The old Z80 had 64kB of address range while Z180 got 1MB.
Most processors that outgrows their normal address ranges do get variants that brings in different tricks to extend the addressable range.
But in the end, it's almost always best to use a processor capable to span all needed memory with the "standard" instruction set. It's just so much simpler to code, compute performance, debug, ... With cheap 32-bit processors readily available, no new projects should really be started with bankswitched 8051 processors.
Hi Per sir,Thank you very much for your valuable time...