Assembly Language An Introduction for ICE4M Teachers

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Assembly LanguageAn Introduction for ICE4M
Teachers

• Imperial Oil Summer Institute
• August 18, 2005
• Presented by
• Michelle Vidberg - michelle_at_acse.net

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Today's Agenda

• Why teach Assembly Language?
• High-level vs. low-level languages
• Choosing a processor
• CPU registers
• Introductory commands
• Types of memory addressing
• Immediate
• Direct
• Indirect
• Sample programs
• Oshonsoft software

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Why Teach Assembly Language?

• Well, because it's in the curriculum...
• ICE4M emphasizes the topics
• - CPU
• - memory registers
• - low-level programming

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By the end of this course, students will

• describe the constructs of a simple assembly or
  machine-level language
• identify similarities and differences among
  memory addressing techniques
• compare high-level and low-level commands that
  perform similar operations.
• trace the execution of simple machine-level
  programs
• write low-level programs
• document all programs to a specified standard.
• For a complete list of expectations, visit
  http//www.edu.gov.on.ca/eng/document/curricul/sec
  ondary/grade1112/tech/tech.htmlICE4M

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High-Level vs. Low-Level Languages

• High-level programming
• - simple code can be easy-to-interpret
• total subTotal salesTax
• put Hello World!
• for count1..10
• put count
• end for

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• Computers don't really understand those
  high-level commands until they're translated into
  machine language via the...
• Compile Process
• Parsing identifies syntax errors
• Code Generation translates high-level commands
  into assembly language (depends on CPU)
• Assembly produces machine language object
  files
• Linking combines object files into an
  executable file

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Teachable Moment !!

• This is a great opportunity to talk about the
  history and development of programming languages.
• How did people write and store big programs
  before the evolution of high-level languages?
• This is of particular relevance to those students
  also enrolled in the ICS courses.

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Machine Language

• Each machine language line of code has 2 parts
• - op-code describes the operation to be done
• - operand identifies the quantity to be
  operated on
• All commands are in binary, shown grouped as
  hexadecimal digits (4 binary digits 1 hex
  digit)
• Still quite difficult to read and understand
• Good applications of binary/hex number systems
  and conversions

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Machine vs. Assembly Language

• Assembly language uses alpha-numeric characters
  and more closely resembles English words or
  acronyms.
• e.g. Move immediate the value 33 into register A
• in machine language 3E 33
• in assembly language MVI A 33
• MVI MoVe Immediate

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8085 Processor

• Intel released several 8-bit microprocessors in
  the 1970s including the 8080, 8085, 8088.
• Simulating a simple processor allows us to show
  the steps, registers, process in a scaled-down
  environment.
• Choose a processor and a corresponding assembly
  language that suits your needs and has the
  resources you need. (8085 used in profiles and
  here)

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8085 CPU Registers
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Introductory Commands

• MVI Put a specified value in a specified
  register.
• e.g. MVI A B8 - put B8 in A register
• MVI B 5F - put 5F in B register

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8
B
MVI A B8
14
F
5
MVI B 5F
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• ADI Add the specified value to register A
• e.g. ADI 4B
• SUI Subtract the specified value from register
  A
• e.g. SUI 12
• INR Increment specified register by 1
• e.g. INR C
• DCR Decrement specified register by 1
• e.g. DCR B

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• LDA Load data into A from given RAM address
• e.g. LDA 14C3
• STA Store data from A into given RAM address
• e.g. STA 2259
• HLT Halt program

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Sample Program 1

• MVI A 33
• INR A
• STA 12C3
• HLT

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3
3
MVI A 33
19
4
3
INR A
20
12C3
4
3
STA 12C3 HLT
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Sample Program 2

• (MVI A 6A)
• (STA 55C1)
• LDA 55C1
• SUI 10
• STA 55C1
• HLT

These first 2 lines are somewhat redundant, but
are needed to load 55C1 with a value!
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55C1
A
6
LDA 55C1
23
A
5
SUI 10
24
55C1
A
5
STA 55C1 HLT
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Types of Memory Addressing 1

• Immediate operand is a specific numerical
  value, not an address in RAM
• e.g. MVI B 52
• ADI 22
• SUI FF

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• Another Immediate Command
• Load Extended Immediate (LXI)
• Loads a pair of registers (BC, DE or HL) with the
  2 bytes specified.
• e.g. LXI D 3F67
• - loads D register with value 3F and
• - loads E register with value 67

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3
F
6
7
LXI D 3F67
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Types of Memory Addressing 2

• Direct operand represents an address in RAM
  (address bus is 16 bits or 2 Bytes)
• e.g. LDA 55A2
• STA AB31

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• Another Direct Command
• Load HL Pair Direct
• LHLD Load value stored at given RAM address
  into L register and load value at next address
  into H register
• e.g. LHLD 45CD
• - load value stored at 45CD into register L and
• - load value stored at 45CE into register H

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• Another Direct Command
• Store HL Pair Direct
• SHLD Store contents of L register at the
  specified RAM address. Store contents of H
  register at the next adjacent RAM address.
• e.g. SHLD 6649
• - store contents of L register at 6649.
• - store contents of H register at 664A.

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Types of Memory Addressing 3

• Indirect operand specifies a pair of registers
  (BC, DE, or HL) where the RAM address of the
  data is stored.

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• An Indirect Command
• Load Accumulator Extended
• LDAX Loads into register A the data stored at
  RAM address specified by pair of registers
• e.g. LDAX D
• Byte from register D and byte from register E are
  put together to make a RAM address. Value stored
  at that RAM address is loaded into register A.

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A2FF
?
?
A
2
F
F
LDAX D
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• Another Indirect Command
• Store Accumulator Extended
• STAX Stores value in register A into the RAM
  address specified by pair of registers
• e.g. STAX B
• Byte from register B and byte from register C are
  put together to make a RAM address. Value from
  register A is stored at that RAM address.

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Sample Program 3

• MVI A 33
• MVI B 89
• MVI C 87
• STAX B
• HLT

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3
3
MVI A 33
37
3
3
8
9
MVI B 89
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3
3
8
9
7
8
MVI C 87
39
8987
3
3
8
9
7
8
STAX B HLT
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Low-Level Repetition Selection

• Introduce concept of looping with JMP
• Every instruction has a memory location (used by
  program counter)
• e.g. JMP 0033
• - means jump to instruction at memory location
  0033
• Must keep track of how much space each
  instruction requires in order to loop to the
  right location

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Conditional Loops

• By comparing two values, we cause flags to be set
  in the status register. Based on the result of
  this comparison we can decide to loop or not.
• 1. Compare another register to A.
• e.g. CMP C
• Sets the flags in the status register as if we
  were subtracting contents of C from A. (Does not
  actually subtract)

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• 2. Compare an immediate value to register A.
• e.g. CPI FF
• Sets the flags in the status register as if we
  were subtracting the value FF from A. (Does not
  actually subtract)

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• Now, looking at the flags in the status register,
  we can tell our program to loop under certain
  conditions. (Each command is followed by the
  memory location to jump to)
• JZ - Jump if Zero
• JNZ - Jump if Not Zero
• JP - Jump if Plus
• JM - Jump if Minus

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Sample Program 4

• MVI A 07
• INR A
• CPI 0B
• JNZ ??
• STA 4400
• HLT

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• 0100 MVI A 07
• 0102 INR A
• 0103 CPI 0B
• 0105 JNZ ??
• 0108 STA 4400
• 010A HLT

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• 0100 MVI A 07
• 0102 INR A
• 0103 CPI 0B
• 0105 JNZ 0102
• 0108 STA 4400
• 010A HLT

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Recommended Software

• Oshonsoft 8085 Simulator
• www.oshonsoft.com
• 100 U.S. / site licence

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Recommended Text

• Networks, Interfaces, Integrated Circuits
• by Graham Smyth and Christine Stephenson
• Holt Software Associates, 2002
• 1-800-361-8324

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