I think there can be a problem where you try to read from one of these ports and since it's cached you're reading a previous value you didn't intend. In memory mapped io there are some problems related to cache, what are those problems? Most of them are well-documented, but every once in a while you'll have some ambiguous edge cases where you're not sure what will happen. To activate memory-mapped I/O, use the mmapsize pragma and set the mmapsize to some large number, usually 256MB or larger, depending on how much address space. In order to know what will or won't happen, you need to be familiar with the platform you're working on. This might relate to your question about caching, but I'm not sure exactly. The volatile keyword when applied to a variable prevents C from optimizing out reads or writes to it. Which is pointless for normal memory, but not for memory-mapped IO. C compilers don't really understand this as a concept and will try to optimize out behavior where you do something that would otherwise be pointless, like read from a memory address you just wrote to. In other words, reading this address and writing to it represent two totally different things. Rather, it tells it "I want an interrupt to occur at the Xth row of pixels" where X is the value written to the port. But if you write to that address, it doesn't cause the video chip to start drawing at that line immediately. write.įor example, many game consoles have a memory-mapped IO port that can be read to tell you which row of pixels on is being drawn right now. To make things even more confusing, a value written to a memory-mapped IO port doesn't always change the value at that address either! It's very common for certain IO ports to have different behavior on read vs.
Mov al, byte ptr this isn't going to make AL = 2 necessarily. VishvadeepGothi Sir has covered the IO Organization: Introduction, IO-Mapped. A port is accessed with an in/ out instruction, it doesn't have a "memory address" that the CPU can see. Memory Mapped: IO device registers are mapped to memory addresses Same CPU instructions to access memory or IO Reuse all memory accessing modes/instructions Reduces available address space for memory Only an issue if address space is small Extra logic required outside of the CPU to access memory or IO based on address supplied. In this live lecture, you will prepare the COA2.0 for GATE CSE/IT 2022 Exam. In port based io if I write to a port will it affect the corresponding memory address or not and vise versa.
0 kommentar(er)
