outline

System Software

• Provides interface between user and "naked machine"
  • machine language instruction
  • load into memory
  • set the PC counter to first instruction
• Should give
  • ease of use (hide details)
  • protection

Virtual machine

• System software is SW, not HW
• Virtual machine is what we wish it looked like
• An intermediary between the hardware and the usr
• Is set of SW services created for the user

Suite of programs (System software)

• Operating system
• Translators
  • compilers, assemblers
• Memory managers
  • Loaders, linkers
• Schedulers
• Utilities
  • text editor
  • calc
  • web browser

Types of System Software

Two Examples

• Assembler
  • translator
• Operating system
  • what should it do

The continuum of Programming Languages

Assembler

• Assembly language
  • early 1950s
  • "Second generation language"
• Assembler (translator
  • Source code --> object code (mach code)
  • one-to-one translation
• Machine specific code

The Translation/Loading/Execution Process

Assembly Language

• Features
  • names for op codes
  • names for memory addresses
  • pseudo-ops to generate data
• Solves many of the shortcomings of machine language

Assembly code

• Instruction format:
  • Label: (binds inst. to mem. address)
  • op code name
  • address field
  • comment (behind "--")
• Our assembly language
  • Figure 6.5
  • N = 4, 2⁴ = 16 inst.set

Typical Assembly Language Instruction Set

• 
• Figure 6.5

Example 1

• 1. Initial valuesR = 20 memory location 80 = 43 memory location 81 = 97
  Operation	Final Contents of Register R	Final Contents of Mem Loc 80	Final Contents of Mem Loc 81
  a. LOAD 80	43	43	97
  b. STORE 81	20	43	20
  c. COMPARE 80	20	43	97
  	(and the GT indicator goes ON)
  d. ADD 81	117	43	97
  e. IN 80	20	Whatever value is entered by the user	97
  a. OUT 81	20	43	97

Example 2

• 2. Initial values memory location 50 = 4
  Operation	Final Contents of Register R
  a. LOAD 50	4
  b. LOAD 4	A copy of the contents of memory cell 4.
  c. LOAD L	Because L is the equivalent to 50, this operation is equivalent to LOAD 50, which is the same as part (a).
  d. LOAD L + 1	A copy of the contents of memory cell 51. This operation means LOAD (L + 1), which is equivalents to LOAD 51. LOAD L + 1 does arithmetic on addresses, not contents.