1. Why are program counter and stack pointer 16-bit registers?
Program Counter (PC) and Stack Pointer (SP) are basically used to hold 16-bit memory addresses.PC stores the 16-bit memory address of the next instruction to be fetched. SP stores address of stack's starting block.
2. What happens during DMA transfer?
During DMA transfers DMA controller takes control of the data transfer, and the processor will carry out other tasks.
3. Define ISR.
An interrupt handler, also known as an interrupt service routine (ISR), is a callback subroutine in an operating system or device driver whose execution is triggered by the reception of an interrupt. Whenever there is an interrupt the processor jumps to ISR and executes it.
4. Define PSW.
The Program Status Word (PSW) is a register which contains information about the current program status used by the operating system and the underlying hardware. The PSW includes the instruction address, condition code, and other fields. In general, the PSW is used to control instruction sequencing and to hold and indicate the status of the system in relation to the program currently being executed. The active or controlling PSW is called the current PSW. By storing the current PSW during an interruption, the status of the CPU can be preserved for subsequent inspection. By loading a new PSW or part of a PSW, the state of the CPU can be initialized or changed.
5. What are the execution modes available in x86 processors?
* Real mode (16-bit)
* Protected mode (16-bit and 32-bit)
* Virtual 8086 mode (16-bit)
* Unreal mode (32-bit)
* System Management Mode (16-bit)
* Long mode (64-bit)
6. What is meant real mode?
Real mode is an execution/operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space, where a maximum of 1 MB of memory can be addressed, direct software access to BIOS routines and peripheral hardware, and no concept of memory protection or multitasking at the hardware level. All x86 CPUs in the 80286 series and later start in real mode at power-on (earlier CPUs had only one operational mode, which is equivalent to real mode in later chips).
7. What is protected mode?
Protected mode allows system software to utilize features such as virtual memory, paging, safe multi-tasking, and other features designed to increase an operating system's control over application software.
When a processor that supports x86 protected mode is powered on, it begins executing instructions in real mode, in order to maintain backwards compatibility with earlier x86 processors. Protected mode may only be entered after the system software sets up several descriptor tables and enables the Protection Enable (PE) bit in the Control Register 0.
8. What is virtual 8086 mode?
Virtual real mode or VM86, allows the execution of real mode applications that are incapable of running directly in protected mode. It uses a segmentation scheme identical to that of real mode, and also uses 21-bit addressing - resulting in linear addressing - so it is subject to paging.
9. What is unreal mode?
Unreal mode, also known as big real mode, huge real mode, or flat real mode, is a variant of real mode. one or more data segment registers will be loaded with 32-bit addresses and limits.
10. What is the difference between ISR and a function call?
ISR has no return value, where as a function call has the return value.