Lecture 1 - 80x86_Structure_and_Flags.pdf

💡Recall

• What’s 8 bits in bytes?
• What’s a bit and what’s a byte?
• Why we need bytes?
• What a half-byte called?
• On 16 bits programming we can access?
• On 32 bits programming we can access?
• What’s ALU?
• How does the microprocessor know that some are instructions and some are operands?

🖋 Notes

x86 microprocessor’s structure

*Addresses (ABUS)

*Data(DBUS)

*Commands(CBUS)

EU (Executive Unit) left side ****

• run the machine instructions by means of ALU (Arithmetic and Logic Unit) component, responsible for the arithmetical logic operations
• doing only integer numbers and +,-,/, operations*
• extra: mathematical coprocessor is used for the other numbers

BIU (Bus Interface Unit) right side

• everything here participates in the address computation
• prepares the execution of every machine instruction:
  • reads an instruction from memory
  • decodes it
  • computes the memory address of an operand, if any
  • output configuration is stored in a 15 bytes BUFFER, from where EU will take it (the BUFFER prepares the next instruction)
• 15 bits is the maximum size that the internal format of an instruction can have

EU and BIU

• work in parallel:
  • EU runs current instruction
  • BIU prepares next instruction
• synchronised actions (the one that ends first waits after the other)

RAM - Random Access Memory

• the access time to one memory area is the same independently of the location of that memory area (randomly far from the beginning of memory)

Computer/processor on N bits

1. software perspective
   • N is the size of a memory word (word size) = the size of the (majority of) the registers
   • "word size" refers to the number of bits processed by a computer's CPU in one go (these days, typically 32 bits or 64 bits)
   • data bus size, instruction size, address size are usually multiples of the word size
   • just to confuse matters, for backwards compatibility, Microsoft Windows API defines a WORD as being 16 bits, a DWORD as 32 bits and a QWORD as 64 bits, regardless of the processor
2. engineering perspective
   • the size of communication buses (channels): ABUS, CBUS, DBUS

Assembler role

• the main role of an assembler is to generate the corresponding bytes

Data type

• structure + associated operations
• in Assembly is very primitive - it means only on thing: the size of the representation