1. ARM Arrays

2. Arrays Array: Block of contiguous values at a known starting address
0x1000 86
0x1004 95
0x1008 73
0x100C 87
0x1010 79
0x1014 98

3. Arrays
Array: Indexed location calculated as: base address + index * elementSize
Array[0]
0x1000 86
Array[1]
0x1004 95
Array[2]
0x1008 73
Array[3]
0x100C 87
Array[4]
0x1010 79
Array[5]
0x1014 98
Array[3] 0x * 4 0x C (12) 0x1012

4. Declaring An Array
Array: Block of contiguous values at a known starting address

5. Little Endian Reminder
Bytes in word numbered right to left
aka “Little Endian”
1000 = 64
1001 = FF
1003 = 00
1012 = 81 +3 +2 +1 +0

6. Little Endian
Array of 4 bytes loaded differently than a word:

7. Array Basics
Always need to load array address

8. Array Basics
Array[0] = Value at array address

9. Array Basics Array[1] = Value at (array address + element size)
4 bytes

10. Array Basics
Array[2] = Value at (array address + 2 * element size)

11. LDR Trick LDR rd, [rn, #immediate] : Calculate rn + immediate
Get value at that memory address into rd

12. Array With Trick
Access as baseAddress + offset

13. Array Basics With Bytes
Always need to load array address

14. Array Basics With Bytes
Load elements as bytes or signed bytes
Store as bytes

15. Array Basics With Bytes
Array[x] = Value at (array address + x * element size)
Element size = 1 byte

16. Array Loops

17. Basic Loop Must Know Keep track of: Size of Array Array Size
Current Location
Index
Total

18. Basic Loop
Test compares
Array Size
Index

19. Basic Loop
Body uses
Current Location
Total

20. Basic Loop
Update changes
Current Location
Index

21. Basic Loop
Keep track of:
Array Size
Current Location
Index
Total

22. Basic Loop
Issue
Lose track of base address

23. Better Loop LDR rd, [rn, rm] : Calculate rn + rm
Get value at that memory address into rd

24. Better Loop Keep track of: Array Size Base Address Index Total
Offset Into Array

25. Better Loop
Indexed Address using
Base Address
Offset Into Array

26. Better Loop
Update
Index
Offset Into Array
Leave baseAddress intact!

27. Best Loop LDR rd, [rn, rm, lsl #x] : Indexed Address
Calculate rn + rm * 2x
Get value at that memory address into rd

28. Best Loop
Keep track of:
Array Size
Base Address
Index
Total

29. Best Loop Indexed Address using
Base Address
Index
Calculate element Location as base + index * size r2 + r3 * 4 Assuming 4 byte data No LSL if accessing byte data!

30. Copy Array Start with Keep Track of Empty Space Array Size
Original Base Address
Empty Base Address
Index

31. Copy Array Indexed Addresses Read from Write to
Original Base Address + 4 * Index
Write to
Empty Base Address + 4 * Index

32. Strings

33. C-String C-String: Character array Ends with null char '\0' or 0
Char = byte
Ends with null char '\0' or 0
Cstring[0]
0x1000
41 (A)
Cstring[1]
0x1001
70 (p)
Cstring[2]
0x1002
Cstring[3]
0x1003
6c (l)
Cstring[4]
0x1004
65 (e)
Cstring[5]
0x1005 00

34. Strings .ascii : allocate array of bytes
.asciz : allocate null terminated array of bytes

35. String Loop – Count e's Start with Null terminated string
Keep Track of
NumberE's
Base Address
Index
Current Char

36. String Loop – Count e's Test Get next char if != 0, continue
Indexed Address base + index

37. String Loop – Count e's Body r4 has current
Conditionally Increment NumberE's

38. String Loop – Count e's
Update
Increment Index