MMIO technology

PMIO What is it? ？

PMIO Its full name is port-mapped I/O, It is used specifically to perform I/O A special class designed for operation CPU Instructions , Such as x86 and x86-64 In the architecture in and out Instructions .I/O The device has an address space independent of general memory , Can pass CPU Additional on the physical interface I/O Pin to achieve . because I/O The address space of is isolated from the address space of main memory , So this is sometimes called isolation I/O.

advantage ：

1. I/O The port address does not occupy memory space ; Use special I／O Command to operate on the port ,I/O Short instruction , Fast execution .
2. Due to the use of special I/O Command access port , also I/O Port address and memory address are separate ,I/O Port address and memory address can overlap , Without confusing each other .

shortcoming ：

1. cpu Separate instructions need to be designed , The implementation is relatively complex
2. Only use EAX、AX and AL Register to control the reading and writing of data , visit I/O The device needs many instructions to complete
3. Special instructions are required to operate I/O Space , Cannot operate with general memory access instructions , High coding cost

In the early cpu The number of bits and memory space are small ,PMIO It has the advantage of not occupying memory space , With the expansion of processor bits and memory space ,PMIO This advantage of architecture is gradually lost ,MMIO Architecture is becoming mainstream .

MMIO What is it? ？

MMIO Its full name is Memory-mapped I/O, Memory mapping I/O For memory and I/O Devices are addressed using the same address space .I/O The memory and registers of the device are mapped into a unified address space .

In this framework , Memory address can refer to physical address RAM Part of , Can also refer to I/O Memory and registers of the device . Used to access memory cpu Instructions may also be used to access peripherals I/O Memory and register space .

MMIO The advantages of

1. MMIO As a different CPU To device communication method , Will not affect the equipment DMA The process
2. MMIO Discarded port I/O The complexity ,cpu Requires less internal logic
3. MMIO Under the architecture , General memory instructions can be used to access I/O peripherals ,CPU All addressing modes of can be used for I/O And memory .

MMIO Examples of use

linux The kernel uses ioremap To map I/O Physical memory , Examples are as follows ：

    gpio_base_addr = ioremap(GPIO_BASE_ADDR, 0x7000);

ioremap When the mapping is complete , Reading and writing I/O Register is consistent with ordinary memory read and write operations , No special code is required , Examples are as follows ：

    unsigned int *base_addr;

    base_addr = (unsigned int *)(gpio_base_addr + offset);
    *base_addr &= ~(0x8 << 7);

MMIO Do not access I/O Registers do not require any special instructions , visit I/O Registers are consistent with accessing normal memory space .