Computer Systems

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Computer Systems

• Overview of
• Instruction Set Architecture

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Overview of ISA

• Instruction
• Operand
• Operation repertoire
• Data types
• Addressing modes
• Registers
• Procedure call mechanism

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Instruction

• An instruction is an op-code followed by
  address(es)
• Address means any address in system
• Registers, memory locations, I/O ports, etc.
• Format
• Layout of bits in an instruction
• Usually more than one instruction format in an
  instruction set
• Length of instruction
• Length of op-code field(s)
• Style
• Number of addresses

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Instruction Style

• Four-address format
• Op-code Src1 Src2 Dst NextInstr
• z ? x op y
• Src1 x, Src2 y, Dst z
• Three-address format
• Op-code Src1 Src2 Dst
• Use program counter for next instruction
• Two-address format
• Op-code Src1 Src2 (and Dst)
• y ? y op x
• Src1 x, Src2 and Dst y
• Example M68000

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Instruction Style

• One-address format
• Op-code Src1
• AC ? AC op x
• Src1 x, Accumulator (AC) is Src2 and DST
• Zero-address format
• Op-code
• Use stack
• push(pop(stack) op pop(stack))
• Post-order expression

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Instruction Style

• Load-store style (3-address format)
• All operations occur in registers
• Three operands except for load/store
• Load load data from memory to register
• Store store data from register to memory
• Op-code r1 r2 r3
• r3 ? r1 op r2
• r1, r2, and r3 are registers
• Example MIPS

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Example I J K

• Four-address format
• ADD J, K, I, NEXT I J K
• next instruction in location NEXT
• Three-address format
• ADD J, K, I I J K
• next instruction in PC
• Two-address format
• MOVE J, I I J
• ADD K, I I K I

How about ADD K, J MOVE J, I
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Example I J K

• Load-store style
• LOAD J,r2 J r2
• LOAD K,r3 K r3
• ADD r1,r2,r3 r1 r2 r3
• STORE r1,I I r1
• M68000 (If both I and J are memory locations)
• MOVE J,D0 D0 J
• ADD K,D0 D0 K
• MOVE D0,I I D0

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More Examples

• I J K L
• I J K L
• I (J K) L
• I (J K) L M
• I J K L M
• I J K (L M)
• I (J K) (L M)

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Conclusion on Instruction Style

• Which one is better?
• Compare
• Instruction counts
• Instruction length
• Memory accesses

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Operands

• Addresses
• Memory locations, registers
• Numbers
• Integers, floating point numbers
• Characters
• ASCII etc.
• Logical Data
• Bits or flags

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Operation Repertoire

• Types of Operation
• Data Transfer
• Arithmetic
• Logical
• Conversion
• I/O
• System Control
• Transfer of Control
• How complex are they?
• RISC (Reduced Instruction Set Computer) vs.
• CISC (Complex Instruction Set Computer)

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Addressing Modes

• Where to find the data items, i.e., operands
• Common addressing modes
• Immediate
• Direct
• Indirect
• Register
• Register Indirect
• Displacement (Indexed)
• Post-increment and pre-decrement
• Etc.

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Registers

• The sketch pads in CPU
• Design isuues
• Number and size
• General purpose
• Specialized
• User visible
• Control and status

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Last but not the least - Endian

• Byte order problem
• What order do we read numbers that occupy more
  than one byte
• e.g. (numbers in hex to make it easy to read)
• 12345678 can be stored in 4x8bit locations as
  follows
• Address Value (1) Value(2)
• 184 12 78
• 185 34 56
• 186 56 34
• 187 78 12
• read top down or bottom up?

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Endian

• The problem is called Endian
• The system on the left has the most significant
  byte in the lowest address
• ? This is called big-endian
• The system on the right has the least
  significant byte in the lowest address
• ? This is called little-endian
• Big-endian examples M68000, Internet
• Little-endian example Intel x86
• Why is it a problem?

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Example Register Organizations