CSCI130

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Lecture 8

• CSCI130
• Instructor Dr. Imad Rahal

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Announcements

• Practice set 3
• Exam 1 review set
• unclaimed quizzes

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(No Transcript)
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Assembly Language

• Machine code consists of the
• binary instructions, data addresses
• can directly be executed by the CPU
• We as humans are not good in working with
  sequences of 1s and 0s
• We prefer to use an English-like style (English
  mnemonics and decimal numbers)
• This is called the assembly language
• It has 1-to-1 correspondence to machine code
• one instruction for every 3-bit Opcode and
  decimal numbers for addresses and data
• Slight other changes

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Assembly Language

• Additional problems addressed
• Most commands like LOAD, STOR, JUMP and ADD
  require memory addresses
• How do we know which address in memory is free to
  use?
• We also assumed that PC will contain 0 initially
• Our program will be loaded to first memory
  locationis this always true?
• What if we have multiple executing programs on
  the same machine?
• Which will get the 0 address then?

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Assembly Language

• In reality, when we program, we dont concern
  ourselves with such low level details
• More on this in the next chapter (OS)
• From now on, we use variables/labels instead of
  addresses and totally forget about memory
• 110 10010 ? ADD Counter which must initialized

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Add all numbers between 1 and 10

• Algorithm
• Sum 0, decrement1, Limit10
• Add limit to sum
• Subtract decrement from Limit
• If Limitgt0
• Go back to step 2
• Display sum
• Stop

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Assemblya nicer way to write the same thing
START LOAD SUM ADD LIMIT STOR
SUM LOAD LIMIT SUB DECREMENT STOR
LIMIT JPOS START LOAD SUM WRITE (
STOR 31) HALT SUM 0 DECREMENT 1 LIMIT
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Changes 1- Use variables for needed data
holders 2- Use labels for jumps 3- WRITE for STOR
31 READ for LOAD 30
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Add all numbers between 1 and N

• Algorithm
• Input N
• Sum 0, decrement1, limit N
• Add limit to sum
• Subtract decrement from Limit
• If Limitgt0
• Go back to step 3
• Display sum
• Halt

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Program to add all numbers between 1 and N
READ //instead of LOAD
30 STOR LIMIT LOOP LOAD SUM
ADD LIMIT STOR SUM LOAD LIMIT SUB
DECREMENT STOR LIMIT JPOS LOOP LOAD
SUM WRITE //instead of STOR 31 HALT SUM
0 DECREMENT 1 LIMIT 0
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Assembly Language

• Computers can only understand machine code
• Programs written in assembly must be translated
  to machine code to be executable by CPU
• The assembler is responsible for that
• Stored in memory and executed before any assembly
  program can be executed
• (english-like) assembly source code ? (binary)
  machine code
• Does a lookup for each word (Opcodes)
• Other things (find empty memory locations for my
  variables)
• The result is called object code

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Program to find the larger of two input numbers

• 0 READ ( LOAD 30) loads 1st number
• 1 STOR X store 1st number at location X
• 2 READ loads 2nd number
• 3 STOR Y store 2nd number at location Y
• 4 SUB X subtract 1st from 2nd
• 5 JPOS Y_LARGER if AC isgt0, then 2nd num is
  larger
• 6 LOAD X otherwise, the 1st num is larger
• 7 WRITE output first number
• 8 HALT program is done
• 9 Y_LARGER LOAD Y
• 10 WRITE Output result
• 11 HALT End program
• 12 X 0 Initialize variables
• 13 Y 0

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More Practice

• Algorithm and program to print all numbers
  between two user-input numbers N1 and N2
  inclusively (downwardsassume N2 gt N1)