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Status: Tags: #cards/cmpt295/assembly Links: x86-64 Assembly - Arrays

x86-64 Assembly Arrays

Principles

1D Arrays

? Remember differences of array cells differ by size of data, not just 1!

• No arrays, just access by displacement of main pointer
• `A[i] => A + i * L
  • A is base memory/address of first point
  • i is index of array
  • L is sizeof(T)

Accessing memory address ?

• these operations give you the same memory address: x + 1, &x[1]

Accessing a value ?

• these operations give you the same value: x[4], *(x+4)

Allow for quick indexing with memory address indexing via movq (%array start, index, char size), %register you want to save to

How to only let loop iterate while i<n ?

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# i in %ecx, N in %esi
cmpl %ecx, %esi # N - i
jle jmp endloop # N <= i

Adding all elements of a char array (version 1):

• Storage
  • %rdi contains starting address
  • %esi contains size (N)
  • %ecx contains array index
  • %al or %ax contains running sum
• Jump if end of array via top jmp

Jump to end, have condition at the end to jump back if not end of array

2D Arrays

Access row i of a 2d array ? A + (R * C * L)

Access row i col j of a 2d array ?

• A + (i * C + j) * L or can distribute L
• C is number of columns A[R][C]
• i is row index
• j is col ind

References:

Created:: 2022-03-19 04:01