C Program Translation

1
C Program Translation

• Translating C programs into assembly

2
Program Translation

• C is closer to assembly than almost any other
  language
• Arithmetic/logic/bitwise operations resemble
  machine operations
• The use of pointers corresponds to memory access
  at machine level
• No high-level language complex elements

3
Program Translation

• For now, use pseudo assembly instructions
• Will see full details of assembly
• Use of registers
• Denote registers as r0, r1, r2,
• Registers can hold byte, half-word (short), word
  (integer)
• r0 holds zero

4
Program Translation

• Memory operations
• Load Load a memory variable at a given memory
  address to a register
• Store Store a register value into a memory
  variable at a give memory address
• No other operations are allowed on memory
  variables
• load r1, a r1 lt mema
• store r2, b memb lt r1
• store r3, 0x40000012 mem0x40000012 lt r3

5
Program Translation

• Data operations
• Add , sub -, mul , div /, and , or , xor ,
  shift-left ltlt, shift-right gtgt
• Can use two registers
• Can use one register, one constant
• Format
• add r1, r2, r3
• add r1, r1, 100

6
Program Translation

• Compare instruction cmp
• Can compare two registers
• Can compare one register, one constant
• Cannot use memory
• cmp r1, r2
• cmp r1, 0

7
Program Translation

• Branch instructions non-sequential control flow.
• (1) Branch condition-when (2) Branch target-where
• Should follow a compare instruction
• beq branch if equal
• bne branch if not equal
• blt branch if less than
• ble branch if less than or equal
• cmp r1, 0
• beq label

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Arithmetic Expressions

• Exercise
• a b
• a 10

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Arithmetic Expressions

• Exercises
• c a b
• e (a b) (c d)

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Translating if statement

• C if statement
• if (cond_expr)
• then_statement
• else
• else_statement

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Translating if statement

• goto version
• t cond_expr
• if (!t) goto false
• then_statement
• goto done
• flase
• else_statement
• done

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Translating if statement

• Exercise
• if (x gt y)
• max x
• else
• max y
• Write goto version
• Translate into pseudo-assembly

13
Translating if statement

• Exercise
• if (x gt 0)
• x 0
• Write goto version
• Translate into pseudo-assembly

14
Translating do-while Loop

• C do-while statement
• do
• body_statement
• while (cond_expr)

15
Translating do-while Loop

• Goto version
• loop
• body_statement
• t cond_expr
• if (t) goto loop

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Translating while Loop

• Exercise Calculate the parity bit of n
• m n
• parity 0
• do
• parity (m 1)
• m gtgt 1
• while (m ! 0)

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Translating while Loop

• C while statement
• while (cond_expr)
• body_statement

18
Translating while Loop

• goto version 1
• loop
• t cond_expr
• if (!t) goto done
• body_statement
• goto loop
• done

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Translating while Loop

• Goto version 2
• goto test
• loop
• body_statement
• test
• t cond_expr
• if (t) goto loop
• done

20
Translating while Loop

• Exercise Calculate the parity bit of n
• m n
• parity 0
• while (m ! 0)
• parity (m 1)
• m gtgt 1

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Translating for Loop

• C for statement
• for (init_expr cond_expr update_expr)
• body_statement

22
Translating for Loop

• Translated to while statement
• init_expr
• while (cond_expr)
• body_statement
• update_expr

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Translating for Loop

• Goto version

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Translating for Loop

• Exercise Calculate the sum of XN
• sum 0
• for (i 0 i lt N i )
• sum Xi
• Write goto version
• Translate into pseudo-assembly

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Memory Pointers

• How to use pointers?
• int pInt m
• pInt m