Transcript pyc05093_Low

Low-level Programming
Language
Review on Machine Cycle
Long long time ago…
Each cycle includes four steps:
1. Fetch
register
2. Decode
3. Execute
4. Store
=> an instruction is executed
(completed)!!
Register
(accumulator)
Instruction
One instruction
Eg. 1011 1001 0011 0100 0001 0010
~One machine cycle
Levels of programming
language
High level language(3GL,4GL)
Low level language
*Assembly language(2GLs)
assembler
*Machine language(1GLs)
CPU
1st-generation languageMachine languages
The format of an instruction
Eg.
Load a value into register 8,
taken from the memory cell 68 cells
after the location listed in register 3 :
[opcode|source|destination| memory address ]
35
3
8
68
100011 00011 01000 00000 00001 000100
Opcode
Operands
* The format of a instruction :
opcode + operand(S)
decimal
binary
1st-generation languageMachine languages
The format of an instruction
*Opcode : (operation code)
-indicate the command (instruction) to
be executed
~ NAME OF THE FUNCTION
*Operands:
-indicate the registers, values or
memory addresses used in the function
~PARAMETER
1st-generation languageMachine languages
Features:
1.
Understood and executed by a computer
at once - binary code =>tedious
2.
Each statement represents one
instruction => lengthy
3 * Machine-dependent
Platform-dependent
(each model of CPUModels
has oftheir
own
CPU (intel)
instructionEg.set)
X86 machine language
1011 1001 0011 0100 0001 0010
1st-generation languageAssembly language
* The format of a instruction also consists :
opcode + operand(S)
Example : Set CX register to be 1234H
Mov
CX, 1234H
Opcode
Operands
Opcode : makes use of
mnemonics (~memory aid)
-”looks” like the meaning
-symbolic and meaningful
Mnemonics
Opcode
Operand
meaning
INP
XXX
Input data to XXX
OUT
XXX
Output content of XXX
STA
XXX
Store accumulator to XXX
LDA
XXX
Load acc. with content of XXX
INC
XXX
Increment content of XXX by 1
DEC
XXX
Decrement content of XXX by 1
JPN
XXX
Jump to address XXX
For your reference only!
1st-generation languageAssembly language
Features:
1.
1 instruction in assembly language =
1 instruction in machine language
=> still lengthy
2. Machine-dependent
=> not portable
3. Easier to be understand and remember
(make use of meaningful codes like mnemonics)
Machine language V.S.
Assembly language
• Similarities
– Lengthy
– Machine independent
• Low portability
– Execute quickly
• Difference
– 2GLs uses meaningful codes
• Easier to remember
• Can be written comparatively efficiently
• Less errors
– 2GLs need to be translated before running
Application??
• Why learning assembly language is
still a good idea?
• system programs
– driver programs
– utility programs