The Basic Instruction Set

1
The Basic Instruction Set

• Handout 4

2

• terms
• instruction set
• machine language
• instruction format
• operand format
• operate format
• addressing modes
• instruction mnemonics
• direct addressing instructions
• arithmetic/logical instructions
• jump instructions
• immediate addressing instructions

3
Operand Type Instructions

• instruction set
• instructions that the machine understands
• also known as machine language
• machine language programs
• programs written using sequences of machine
  language instructions
• instruction format
• CUSP instructions are 24 bits wide, and have two
  formats
• operand format and operate format
• operand format
• OOMAAA
• OO is an 8-bit opcode (identifies operation to be
  performed)
• AAA is a 12-bit addressy thing
• M is a 4-bit addressing mode (tells how to use
  the addressy thing to get the operand)

4
Operand Type Instructions Examples And Such

• 002004
• opcode 00 (tells the control unit to load a
  value into ACC)
• addressing mode 2 (tells CU that the addressy
  thing is actually a memory address)
• address 004
• 002004 is equivalent to the following two forms
• load into ACC the contents of memory location
  004
• LDA 004 (this form is called a mnemonic)
• how many operand type instructions are there?
• depends on the number of bits in the opcode
• FF is not allowed to begin an operand type
  instruction
• this leaves 255 possibilities of which 56 are
  valid

5
Operate Type Instructions

• these do not need an operand to be specified
• examples
• FFF020 tells the control unit to negate the
  contents of ACC
• if ACC is 000 001, it becomes FFF FFF
• FFF021 tells the control unit to invert the
  contents of ACC
• if ACC is 000 001, it becomes FFF FFE
• in both examples, ACC is the only operand and is
  implicitly specified
• the most significant 12 bits are always 1
• operate instructions always start with FFFOOO
• OOO can have 2124096 possible combinations
• only 38 of them represent valid operate
  instructions

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Addressing Modes

• tells the CU how to use the addressy thing to get
  the operand
• two major addressing modes
• direct the addressy thing is the address of a
  memory location from where the operand can be
  retrieved
• 002004 OOMAAA LDA 004
• M 2 tells the CU that direct addressing mode is
  being used, and that the operand is available at
  memory location 004
• immediate the addressy think is actually the
  operand
• 000004 OOMAAA LDA 004
• M 0 tells the CU that immediate addressing mode
  is being used, and that the operand is available
  immediately and is equal to 004
• immediate mode is indicated in the mnemonic by a
  symbol

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Instruction Mnemonic Rules For CUSP

• operand type instructions
• format is XXXC1 C2value
• XXX is the opcode mnemonic such as LDA
• value is the addressy thing
• C1 and C2 determine which addressing mode is
  being used
• 10 addressing modes are used in CUSP
• if C2 and C1 are both blank, it is direct
  addressing mode
• if C2 is blank and C1 is , it is immediate
  addressing mode
• operate type instructions
• format is XXXX
• XXX is the opcode mnemonic such as NEGA or COMA

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Assembler

• mnemonic representation of instructions is easier
  to read
• must be converted to the hexadecimal
  representation before it can be stored in the
  computer
• the software that converts mnemonics into
  hexadecimal is called an assembler

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A Simple Program
At the start of the program, PC 000.
Memory (after)
Addr Contents000 002004001 102005002 0420
06003 FFFFFF004 000011005 000008006 00
0019
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Program Flow Control Instructions

• sometimes next instruction executed is not the
  next stored instruction
• such transfer of flow control is supported
  through flags and jump instructions
• flags
• EQ, OV and LT are set or cleared as a result of
  execution of most of the instructions
• EQ is set if the result of an operation results
  in 0
• OV is set if an operation causes an overflow
• LT is set if the result of an operation results
  in a negative numberi.e., if the result has a 1
  in the leftmost bit position

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Flag Modification Example 1

• Assume thatACC 000 00CMem010 FFF
  FF2IR ADA 010
• the instruction to be executed is ADA 010, i.e.,
  add the contents of memory location 010 to the
  contents of ACC and store the result in ACC
• new value of ACC 12-14 -2
• since the result
• is less than 0, LT flag will be set to 1
• is not zero, EQ (or equal-to-zero) flag will be
  set to 0
• has not caused an overflow, OV flag will be set
  to 0

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Jump Instructions

• JMP AAA (or 402AAA in hexadecimal)
• meaning load the PC with address AAA, or PC
  AAA
• no flags affected
• JEQ AAA (or 482AAA in hexadecimal)
• meaning load the PC with address AAA if EQ 1
• no flags affected
• JLT AAA (or 4A2AAA in hexadecimal)
• meaning load the PC with address AAA if LT 1
• no flags affected
• CMA AAA (or 202AAA in hexadecimal)
• sets EQ and LT flags after comparing MemAAA and
  ACC
• EQ (ACC MemAAA)
• LT (ACC lt MemAAA)

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An Example CUSP Program With Jumps
14
The Data Transfer Instructions The MOVes
15
The Control Transfer Instructions Jumps and
Branches
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The Arithmetic Instructions