Device Access

Device Access

This post is regarding a presentation that I did in SPO600 where I talked about device access in low level languages.

Access using I/O Ports

I/O ports: These look just a memory cell to a computer but they connect any data written or read from it to a device that is connected to the computer.

  MOV DX, 0487 ; Port number.
  MOV AL, 1    ; Number to write to the port.
  OUT DX, AL   ; Write to the port.
  IN DX, AL    ; Read from the port.

In this case DX takes in the port number and AL takes in the number you are writing to the port. OUT will write the number stored in AL to DX and IN will read from the device and store results in AL.
Danger:
Accessing devices in this way is dangerous. Every port cannot be accessed in the same way, Some ports are read only, and some are write only and some are both. If you don’t know exactly what port you are using you risk damaging any device that is accessing that port.

Access using Interrupts

An alternative method is using software interrupts to gain access to a device, This is often used in more complex devices such as the mouse in order to simplify accessing it.

  MOV AX, 0 ; Access subfunction 0 of int 33h 
  INT 33    ; Make the interrupt call

In the Example we use Interrupt 33 which provides access the mouse, and moving a number into AX will allow you to access a specific function of the mouse (function 0 returns a value which indicates if a mouse has been detected/installed). There are many, many more sub-functions available here that allow you to access many different parts of the mouse.

Platform / architecture Issues

I/O and device instructions are very processor dependent, due to the details of how a processor moves data in and out, most of the source code is coded specifically for a platform. For example in the linux kernel they have many different files including io.h that are stored in folders specific to the architecture.
Arm: io.h
x86: io.h

Conclusions

I was unable to find to much information about this topic and Chris Tyler, my professor for SPO600, pointed out to me that this is because all the device access SHOULD be handled by the operating system unless you are writing for device drivers or doing embedded programming.

Resources

• 1 A portion of a book explaining IO ports and interrupts
• 2 A tutorial explaining IO ports and software interrupts
• 3 One chapter of a book about device driver programming detailing IO ports and how they are used with devices

Thanks for reading