Chaptr 6 System Software n301.tplt 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

Operation	Final Contents of Register R	Final Contents of Mem Loc 80	Final Contents of Mem Loc 81
1. Initial valuesR = 20 memory location 80 = 43 memory location 81 = 97
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

Operation	Final Contents of Register R
2. Initial values memory location 50 = 4
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.