![]() ![]() Then somebody could write decoder logic that directed some subset of that range to I/O devices (and the rest to actual memory). If they'd chosen to, they could pretty easily have designated that pin as (for example) A20, so the chip would have one extra address pin, allowing it to address 2 megabytes of memory. Then the IO/M pin (with a bar over the M) determines whether a particular read/write goes to memory or an I/O device (I/O device when it's high, memory when it's low). This goes for both I/O devices and memory. With this, AD0 through AD7 carry the data and (depending on bus phase) the 8 least significant bits of the address. For example, the original 8088 had a pinout like this: In practice, even a CPU that treats the two as separate doesn't usually separate them completely. The basic idea is pretty simple: a chip can have one bus to connect to memory, and a second bus to connect to I/O devices-or it can share a single bus between the two. So basically "address space" means that both the memory and the IO devices will be in the same "pool" of addresses, and so the CPU can treat the memory and the IO devices as one logical memory chip. ![]() That the IO devices uses memory-mapped IO. If the monitor have 12 KB of memory, the OS will pick for example Range allocated for the memory (30394 to 2147514042), so for example That will be allocated for the IO devices will not be in the same Have.), now the important thing here is that the memory addresses The memory (it will ask each IO device controller how much memory the IO device The OS will do the same thing with the IO devices as it did with.Requests on the memory addresses from 30394 to 2147514042. GB), now the OS will tell the memory controller to respond to OS will pick a range of addresses that consists of 2 GB, let's assume The OS will ask the memory controller for much memory the memoryĬhip have, let's assume the memory chip have 2 GB of memory.This is what happens when the OS starts running: I want to know what does "address space" means. Processors (most notably the x86 family) have separate read and writeĮlectrical lines for I/O ports and special CPU instructions to access Memory and, therefore, deserve a separate address space. While some CPU manufacturers implement a single address space in theirĬhips, others decided that peripheral devices are different from ![]()
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