CS 301 Fall 2001 Chapter 2

1
CS 301 Fall 2001 Chapter 2

• Slides by Prof. Hartman, following IBM PC
  Assembly Language Programming by Peter Abel

2
Operating Systems

• File management
• Input/Output
• Program Loading
• Memory Management
• Interrupt Handling

3
BIOS boot process

• When you turn on the computer it resets, clears
  all memory to 0, performs a parity check of all
  memory, sets CSFFFF0H and IP0.
• BIOS (in ROM) starting at FFFF0H checks ports to
  identify and initialize devices and then
  establishes the Interrupt Vector Table and the
  BIOS data areas. It then accesses a bootstrap
  loader from the disk for the operating system.
• Programs access devices through the OS, through
  BIOS, or directly.

4
System loader

• Executable programs are .EXE or .COM
• .COM programs have one segment for code, data,
  and the stack
• .EXE programs have separate segments.

5
Loading a .EXE program

• The system loader
• Accesses the .EXE program from the disk
• Constructs a 256 byte program segment prefix
  (PSP)
• Stores the program in memory following the PSP
• Loads the address of the PSP into DS and ES
• Loads the address of the code segment into CS and
  sets IP to the offset of the first instruction
  (usually zero).
• Loads the address of the stack into SS and sets
  SP to the size of the stack.
• Transfers control to the program

6
The Stack

• Three main uses
• Saving return addresses for subroutines
• Passing data to subroutines
• Temporarily saving contents of registers so the
  program can use those registers for computation.

7
The Stack 2

• SS contains the address of the beginning of the
  stack, SP contains the size of the stack (and
  thus points to one address past the end of the
  stack)
• The stack begins storing data at the highest
  location in the segment and stores data downward
  through memory using PUSH and POP (and other)
  instructions. (See example, page 25.)

8
Stack Instructions

• PUSH, POP to and from general register, segment
  register, or memory
• PUSHA, POPA save and restore contents of all
  general purpose registers (16 bytes)
• PUSHF, POPF save and restore contents of the
  flags
• PUSHAD, POPAD save and restore contents of all
  extended registers. (32 bytes)

9
Instruction Execution and Addressing

• Processor steps in executing an instruction
• Fetch next instruction from memory and place it
  in instruction queue
• Decode the instruction calculate addresses for
  memory references, deliver data to the ALU,
  increment IP
• Execute the instruction perform the operation,
  store results in register or memory, set any
  required flags.

10
Pipelining

• The Pentium architecture overlaps operations, as
  in this (simplified) example

11
Instructions and Operands

• Instructions are operations the CPU can perform
  such as PUSH, MOV, etc.
• Each instruction can have zero, one, two, or
  three operands what it operates on.

12
Example of Assembly code

• Note the difference between this and the example
  on page 29 of the text. In the third line,
  WORDX is used (NASM syntax) rather than WORDX
  (MASM syntax)