Introduction to 8086 Assembly

Transcription

1 Introduction to 8086 Assembly Lecture 6 Working with memory

2 Why using memory? Registers are limited in number and size Program data Program code etc.

3 The data segment dd 1234 dw 13 db -123

4 The data segment But how to access data? dd 1234 dw 13 db -123

5 How to access data? Labels! l1 dd 1234 dw 13 db -123

6 How to access data? l1 dd 1234 memory1.asm global asm_main asm_main enter 0,0 pusha mov eax, l1 mov eax, [l1]

7 Reading data from memory l1 dd 1234 global asm_main asm_main enter 0,0 pusha mov eax, l1 mov eax, [l1]

8 Data labels vs. Code labels memory2.asm l1 dd 1234 global asm_main asm_main enter 0,0 pusha mov eax, asm_main mov eax, [asm_main]

9 Data labels vs. Code labels memory2.asm l1 dd 1234 global asm_main asm_main enter 0,0 pusha mov eax, asm_main mov eax, [asm_main]

10 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B"

11 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B" data size (not data type!)

12 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B" db 1 byte dw 2 bytes dd 4 bytes dq 8 bytes dt 10 bytes data size (not data type!)

13 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B" data size mov eax, l1 mov eax, l2 mov eax, l3 mov al, [l8] call print_char

14 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B" data size mov eax, l1 mov eax, l2 mov eax, l3 mov al, [l8] call print_char

15 Definitions in the book Carter, PC Assembly Language, 2007.

16 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B" Where are the variables? data size

17 Putting data in memory l1 db 123 l2 dw 1000 l3 db 11010b l4 db 12o l6 dd 1A92h l7 dd 0x1A92 l8 db 'A' l9 db "B" High-level languages (like C) Variables Low-level language (assembly) Labels (Symbols, Addresses) data size

18 Putting data in memory l1 dd 11, 12, 13, 14, 15, 16 mov eax, [l1] mov eax, [l1+1] mov eax, [l1+2] mov eax, [l1+3] mov eax, [l1+4]

19 Putting data in memory l1 dd 11, 12, 13, 14, 15, 16 mov eax, [l1] mov eax, [l1+1] mov eax, [l1+2] mov eax, [l1+3] mov eax, [l1+4]

20 Putting data in memory l1 dd 11, 12, 13, 14, 15, 16 l2 dd 8, 8, 8, 8, 8, 8, 8, 8, 8 l3 times 9 dd 8 l4 resd 9 l5 resw 18 l6 resb 36

21 Argument types l1 dd 11, 12, 13, 14, 15, 16 mov eax, [l1] mov [l1+4], ebx mov eax, ebx mov eax, l1 mov eax, 123 memory register immediate (constant)

22 Invalid mem,mem assembly commands l1 dd 11, 12, 13, 14 l2 dd 100 mov [l1], [l2] add [l1], [l2] sub [l1], [l2] adc [l1], [l2] sbb [l1], [l2] cmp [l1], [l2] and [l1], [l2] or [l1], [l2] xor [l1], [l2]

23 Invalid mem,mem assembly commands l1 dd 11, 12, 13, 14 l2 dd 100 mov [l1], [l2] add [l1], [l2] sub [l1], [l2] adc [l1], [l2] sbb [l1], [l2] cmp [l1], [l2] and [l1], [l2] or [l1], [l2] xor [l1], [l2] invalid!

24 Operation size l1 dd 11, 13, 14 l2 dd 100 mov [l1], eax add eax, [l2] sub eax, 44 mov [l1], 44

25 Operation size l1 dd 11, 13, 14 l2 dd 100 mov [l1], eax add eax, [l2] sub eax, 44 mov [l1], 44 mov dword [l1], 44 mov word [l1], 44 mov byte [l1], 44

26 Assembly command formats List of x86 instructions http// https//c9x.me/x86/

27 Storing multibyte data a dd mov eax, 0 mov al, [a] endianness1.asm

28 Storing multibyte data a dd FF A3 64 mov eax, 0 mov al, [a] endianness1.asm

29 Storing multibyte data a dd mov eax, FF A3 64 MSB LSB mov al, [a] endianness1.asm

30 Storing multibyte data a dd mov eax, decimal 00 FF A3 64 MSB LSB mov al, [a] endianness1.asm

31 Storing multibyte data Memory a dd FF A3 64 MSB LSB decimal mov eax, 0 mov al, [a] endianness1.asm

32 Storing multibyte data Memory a dd FF A3 64 MSB LSB decimal mov eax, 0 mov al, [a] endianness1.asm

33 Storing multibyte data Memory a dd FF A3 64 MSB LSB decimal mov eax, 0 mov al, [a] endianness1.asm

34 Storing multibyte data Memory a dd FF A3 64 MSB LSB decimal mov eax, 0 mov al, [a] endianness1.asm

35 Storing multibyte data Memory a dd FF A3 64 MSB LSB decimal mov eax, 0 mov al, [a] endianness1.asm

36 Storing multibyte data Memory a dd FF A3 64 MSB LSB decimal mov eax, 0 mov al, [a] endianness1.asm

37 Endianness a dd mov eax, 0 mov al, [a] decimal 00 FF A3 64 MSB LSB Memory Memory mov al, [a+1] mov al, [a+2] mov al, [a+3] endianness2.asm Big Endian Little Endian (e.g. x86)

38 Endianness a dd mov eax, 0 mov al, [a] decimal 00 FF A3 64 MSB LSB Memory Memory mov al, [a+1] mov al, [a+2] mov al, [a+3] endianness2.asm Big Endian Little Endian (e.g. x86)

39 Checking endianness in C unsigned int a = 0x1A2B3C4D; printf("%x\n", a); p 1032 p 1032 unsigned char *p = (unsigned char *)(&a); printf("%x\n", *p); printf("%x\n", *(p+1)); printf("%x\n", *(p+2)); printf("%x\n", *(p+3)); a A 2B a D 3C 3C 2B 4D 1A test_endianness.c Big Endian Little Endian (e.g. x86)

40 Checking endianness in C unsigned int a = 0x1A2B3C4D; printf("%x\n", a); p 1032 p 1032 unsigned char *p = (unsigned char *)(&a); printf("%x\n", *p); printf("%x\n", *(p+1)); printf("%x\n", *(p+2)); printf("%x\n", *(p+3)); a A 2B a D 3C 3C 2B 4D 1A test_endianness.c Big Endian Little Endian (e.g. x86)

41 Change endianness xchg ah, al ; 16 bit bswap eax ; 32 bit bswap rax ; 64 bit (x64 only) 1A 2B 4D 3C 3C 2B 4D 1A Big Endian Little Endian (e.g. x86)