Hardware basics CS 2XA3 Term I, 2020/21
Outline Basic architecture Byte order – Endianess NASM 64 Hardware platform
Basic architecture fetch instructions data memory bus 64 bits 64 bits execute registers 64 bits CPU memory Central Processing Unit store
◮ Average Desktop Processor speed is 1.5 to 2.5 GHz ◮ Average Laptop Processor Speed is 1.0 GHz ◮ One gigahertz is equal to 1,000,000,000 Hz. ◮ Gigahertz is used to measure CPU clock speed.
Byte order – Endianess Whenever a number is represented by more than one byte, the question arises as to the order in which the bytes are arranged. ◮ If the most significant bits come first, that is, are stored at the lowest memory address or at the first location in the file, the representation is said to be big-endian . ◮ If the least significant bits come first, the representation is said to be little-endian .
bit pattern 00001101 00000110 11111111 00000010 hex pattern 0D 06 80 03 decimal value 13 6 128 3 big-endian 13 ∗ 256 3 + 6 ∗ 256 2 + 128 ∗ 256 + 3 = 218 , 529 , 795 3 ∗ 256 3 + 128 ∗ 256 2 + 6 ∗ 256 + 13 = 58 , 721 , 805 little-endian
Most computers these days are little-endian since the Intel and AMD processors that most PCs use are little-endian. Big-endian is the most common format in data networking; for this reason, big-endian byte order is also referred to as network byte order . Netwide Assembler (NASM for short) 64 being an x86-64 platform (Intel) is little-endian.
NASM 64 Hardware platform There are 16 64-bit registers: RAX, RBX, RCX, RDX, RSI, RDI, RSP , RBP , R8, ..., R15. The registers the callee (called function) must preserve for the caller are referred to as callee-saved registers. 64-bit 32-bit 16-bit 8-bit register description subreg subreg subreg(s) RAX EAX AX AL , AH Used for return values from functions RBX EBX BX BL , BH Callee-saved register. Must be saved/restored ! RCX ECX CX CL , CH Some instructions also use it as a counter. RDX EDX DX DL , DH AX RAX AL AH EAX
64-bit 32-bit 16-bit 8-bit register description subreg subreg subreg(s) RSI ESI SI SIL Used to pass function arg. #2 in 64-bit Linux. RDI EDI DI DIL Used to pass function arg. #1 in 64-bit Linux. RSP The Stack Pointer . Points to the top of the stack. ESP SP SPL Do not use without fully understanding its role. RBP EBP BP BPL The Base Pointer . Points to the “base”. Do not use without fully understanding its role. DI RDI DIL EDI
64-bit 32-bit 16-bit 8-bit register description subreg subreg subreg(s) R8 R8D R8W R8B R9 R9D R9W R9B R10 R10D R10W R10B R11 R11D R11W R11B R12 R12D R12W R12B Callee-saved register. Must be saved/restored ! R13 R13D R13W R13B Callee-saved register. Must be saved/restored ! R14 R14D R14W R14B Callee-saved register. Must be saved/restored ! R15 R15D R15W R15B Callee-saved register. Must be saved/restored ! R8W R8 R8B R8D
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