Brookshear Machine

1
Brookshear Machine

• Programming
• in
• Machine Language

2
Brookshear Machine

• 16 general-purpose registers
• numbered 0 through 15
• Hex notation numbered 0 through F
• 256 byte-size main memory cells (i.e., 8 bits
  each)
• numbered 0 through 255
• Hex notation numbered 00 through FF
• 12 simple instructions
• encoded using 16 bits (2 bytes) per instruction
• Hex notation 4 hex digits per instruction
• One hex digit for op-code
• Other three hex digits for Operands

3
Brookshear Machine Architecture
4
Brookshear Instruction Format
Actual 16-bit patterns per instruction
0011
0101
1010
0111
Hex form (4 bits)
3
5
A
7
Operand
Op-code
5
Brookshear Machine Instructions

• Op-code Operation
• 1 LOAD contents
• 2 LOAD value
• 3 STORE
• 4 MOVE
• 5 ADD 2s complement
• 6 ADD floating point
• 7 OR
• 8 AND
• 9 XOR
• A Rotate
• B Jump
• C Halt

6
Exercise trace by hand

• Using Appendix C, trace the following machine
  instruction by hand and write the results, given
  the contents of main memory as in the table
• 156C
• 166D
• 5056
• 306E
• C000

Main memory
7
BSML In-class Exercises

• Textbook page 81 1, 7
• Textbook page 86 1, 2
• Textbook page 100 8

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Bit-wise Logical Operations

• A byte can be seen as a set of 8 Boolean
  variables (each bit is one variable)
• Logical instructions like AND, OR, XOR, etc. are
  performed by aligning the bytes and performing
  the logical operations on the corresponding bits,
  one by one.

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Bit-wise Logical Operations

• AND OR
• 1001001 11100001
• 1110001 10001100
• 1000001 11101101
• XOR
• 100101001
• 111001101
• 011100100

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Masking

• To test the individual pattern of bits in a given
  string of bits
• Using particular sequences of bits as the Mask,
  along with the appropriate logical operation, a
  programmer can determine the values of individual
  bits in a byte
• The sequence of bits that are used to examine a
  particular bit is known as the mask

11
Masking Techniques Reading

• Reading a bit in a bit string is done by masking
  away the bits we are not interested in
• AND operator along with a bit mask of 1 in the
  position we want to read will leave the
  interesting bit and mask away the others
• Example Suppose you want to determine if a
  number is odd or even. The low-order bit
  (rightmost binary digit) is 1 in an odd number
  and 0 in an even number. A mask of 0001 with
  AND operator will test the last bit
• Even Number Odd Number
• 100101010 1010100101
• AND 000000001 AND 0000000001
• 000000000 0000000001
• By examining the result of the masking operation,
  you can determine the number as odd or even If
  the result is 0, it is even

12
Question

• Suppose you have an 8-bit string (a byte) that
  is in 2s complement notation, sitting in a
  memory cell. You want to determine if it
  represents a positive or negative number. What
  bit-mask would you use? What Logical operation
  would you use?

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Masking Techniques Setting

• Setting (set to 1) a bit in a bit string is done
  by
• an OR operation with 1 in the position we want to
  set to 1, 0 in the other positions leaving the
  other bits unchanged.
• Example Suppose you want to set the high-order
  bit to a 1 in a given bit string
• 00100110
• OR 10000000
• 10100110

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Question

• Suppose you want to set the 2nd bit from left in
  a given bit string to 1. What is the bit mask you
  would use? and what is the operation to achieve
  the result?
• i.e., given the bit string 10000010, we want the
  result to be 11000010.

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Masking Techniques Re-Setting

• Re-Setting (set to 0) a bit in a bit string is
  done by
• AND with 0 in the bit position that needs
  resetting, and 1 in the other positions in order
  not to change the other bits
• Example Suppose you want to reset the high-order
  bit to a 0 in a given bit string
• 10100110
• AND 01111111
• 00100110

16
Converting ASCII Case

• Purpose Work with bit masks
• Who Groups of 2 3
• Task Develop a mask and select operator to
  convert uppercase ASCII characters to lowercase,
  e.g. A to a, B to b, etc.
• Product Mask and operator. Spokesperson will
  present groups finding
• Time 5 minutes
• (See Appendix A for ASCII code reference )

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ASCII code reference
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Algorithm for converting ASCII case

• First, pick any alphabet and notice the bit
  pattern match for lower and upper case in ASCII.
  Do the same with as many letters of the alphabet
  as you want, till you notice the pattern
• Then, use appropriate mask and logical operator
  to set the appropriate bit for the lowercase
  ASCII bits

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Combining Nybbles

• A Nybble (or, nibble) is a half-byte 4 bits
• Purpose Develop a sequence of logical operations
  using bit masks
• Who Groups of 2-3
• Task You are given two bytes. You must create a
  third byte that combines the first half of the
  1st byte (4 bits) with the last half of the 2nd
  byte (4 bits).
• For example, given 01101001 and 11100111, the
  answer would be 01100111. Devise a sequence of
  logical operations using bit masks to do this.
• Product Masks and operations
• Time 5 minutes

20
Algorithm for Combining Nybbles

• Using appropriate mask and logical operator, read
  the first 4 bits of first byte and store the
  result as result1
• Using appropriate mask and logical operator, read
  the last 4 bits of the second byte and store the
  result as result2
• Combine result1 and result2 using appropriate
  logical operator to set the final result

21
Rotation right and leftwrap around

• Rotate left 1 position
• 01001010 ? 10010100
• Rotate left 1 position again
• 10010100 ? 00101001

• Rotate right 1 position
• 01001010 ? 00100101
• Rotate right 1 position again
• 00100101 ? 10010010

22
Shift right and left

• Shifting is similar to rotation, except the bits
  fall off the end instead of wrap around and
  you fill in the gap with 0.
• Shift right 1 position
• 11001010 ? 01100101
• Shift right 1 position again
• 01100101 ? 00110010
• Shift left 1 position
• 11001010 ? 10010100
• Shift left 1 position again
• 10010100 ? 00101000

Left fill with 0
Right fill with 0
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Arithmetic Shift

• A special form of shift, except the sign bit is
  preserved
• Arithmetic shift to the right by one position
• 11001001 ? 10100100

Preserve the sign bit
Left fill with 0 after shifting
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No Shift Instruction?

• Extra Credit Question
• Notice that the Brookshear machine does not have
  a SHIFT instruction, although there is a ROTATE
• How can a SHIFT be accomplished in the Brookshear
  machine?
• Think about it later. Write your answers on a
  sheet of paper. Demonstrate your answer with two
  examples to get credit. You can turn it in
  anytime before final exam.

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Hand Trace

• Purpose Trace some machine language programs
• Who Groups of 2-3
• Task What is the result of executing the
  following programs?
• Product Describe the state of the machine after
  execution (memory and register contents).
  Spokesperson presents results.
• Time 10 min

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Hand Trace Exercises

• Program 1 what is the contents of cell address
  10 when program halts?
• 2005
• 2106
• 5201
• 3210
• C000
• Program 2 Suppose the cell addresses 08-0F
  contain the following data
• Address Contents
• 08 2A09 3D0A 140B FF0C F00D F10E F2
  0F F3

• Program 2 continued
• Trace the following instructions and explain what
  the program does.
• 1108
• 1209
• 130A
• 5412
• 5443
• 340A
• 4043
• 9413
• 340B
• C000
• What are the contents of cells 0A and OB when
  program halts?

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Program Execution

• Use the Brookshear Machine Simulator at the
  following site
• http//spot.pcc.edu/spreuitt/CS160/Simulators/BSM
  achine/machinegui.html

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Simulator trace Exercise 1 (5 mins)

• Address contents
• 00 20
• 01 00
• 02 21
• 03 01
• 04 23
• 05 01
• 06 12
• 07 12
• 08 B2
• 09 10
• 0A A1
• 0B 07
• 0C 50
• 0D 03
• 0E B0
• 0F 08
• 10 C0
• 11 00

• Run this program stored in cells 00 through 12
  with different value in location 12
• 04
• 02
• What does this program do?

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Simulator Trace Exercise 2 (5 min)
Try the following values in cells 20 and 21. In
each case, write down the contents of cell 22
when the program completes execution. Case
1 cell 20 contains 04 and cell 21
contains 02 Case 2 cell 20 contains 05 and
cell 21 contains 02 What does this program do?

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Write a Program in Brookshear Machine Language
(BSML)

• Purpose Write a BSML program
• Who Groups of 2
• Task Write a program to add three numbers
  together. The numbers are stored at memory cell
  locations 00, 01, and 02. The result should be
  stored at location 03.
• Product A program to test on the simulator
• Time 5 minutes

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One Answer

• 00 ??
• 01 ??
• 02 ??
• 03 ??
• 04 10
• 05 00
• 06 11
• 07 01
• 08 50
• 09 01
• 0A 11
• 0B 02
• 0C 50
• 0D 01
• 0E 30
• 0F 03
• 10 C0
• 11 00

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Write a BSML Program Nybbles

• Purpose Write a BSML program
• Who Groups of 2
• Task Develop a machine language program to
  combine nybbles. The given bytes are stored in
  locations 00 and 01, and the result should be
  stored in location 02.
• Product Program to test on the simulator
• Time 10 minutes

33
Write a BSML Program ASCII case

• Purpose Write in BSML program
• Who Groups of 2
• Task Write a program to convert uppercase to
  lowercase for an ASCII character stored in a
  memory location
• Product Program to test on the simulator
• Time 10 minutes