Language of the Computer

Instructions in Memory(1)

• Instructions are the language of computer
  • vocabulary
  • sentences with a formal grammer
• Same ISA family supports same instructions
  • supports backward compatibility (하위호환성 지원)
• Different computers have different instruction sets
  • but with many aspects in common
• Early computers had very simple instruction sets
  • simplified implementation
• Ever since, the ISA have tended to be complicated
  • called CISC (Complex Instruction Set Computer)
  • 고성능이지만 높은 파워 소모 -> 효율적 시스템 구성이 어려움
• Many modern computers have simple instruction sets
  • called RISC (Reduced Instruction Set Computer)
  • 하나의 명령어가 수행할 수 있는 일의 크기를 줄여 각기 비슷한 일을 수행
  • 하나하나 성능은 떨어질 수 있으나 효율적 ISA 구축이 가능, 낮은 파워 소모

Instructions in Memory(2)

• 1. Instructions are represented as numbers and, as such, are indistinguishable from data (데이터와 명령어는 구분이 안됨 - 숫자로 표현되기 때문)
• 2. Programs are stored in alterable memory(that can be read or written to) just like data

• Stored-program concept

  • Programs can be shipped as files of binary numbers - binary compatibility
  • Computers can inherit ready-made software provided they are compatible with an existing ISA

Subtract Example

• C code : f = (g + h) - (i + j);
• translates into the following assembly code

  add t0, g, h
  add t1, i, j
  sub f, t0, t1

• Each version may produce a different result because floating-point operations are not necessarily associative(결합법칙) and commutative(교환법칙) ... more on this later

Operands

• In C, each "variable" is a location in memory

• In hardware, each memory access is expensive - if variable a is accessed repeatedly, it helps to bring the variable into an on-chip scratchpad and operate on the scratchpad (registers)

• To simplify the instructions, we require that each instruction (add, sub) only operate on registers

• Note : the number of operands (variables) in a C program is very large; the number of operands in assembly is fixed..

  • there can be only so many scratchpad registers
  • 레지스터 개수도 정해져 있음

Registers

• The MIPS ISA has 32 registers (x86 has 8 registers) - Why not more? Why not less? Cost..
• Each register is 32-bit wide (modern 64-bit architectures have 64-bit wide registers)
• A 32-bit entity (4bytes) is referred to as a word
• To make the code more readable, registers are partitioned as $s0-$s7 (C/Java variables), $t0-$t9 (temporary variables)