程式語言
• 語言是用來溝通的工具, 人類有人類的語言,
電腦也有電腦的語言。我們必須將要電腦
做的事情,以電腦了解的語言來表達, 電腦才
能替我們完成工作。要和電腦溝通就要用
程式語言 (Programming Language)。
• 程式就是利用程式語言的敘述, 遵照其一定
的規則及所要處理工作的順序, 編排而成的
一連串命令。
7-1
程式語言
• 依電腦可接受的程度來區分, 程式語言可分
為以下幾種：
7-2
機械語言
• 電腦所使用的語言就是機械語言 (Machine
Language) , 它們都是由 0 與 1 數字所組成,
跟人類的思考模式完全不同, 使得學習與撰
寫上都十分困難, 更別說是拿來發展程式。
7-3
組合語言
• 組合語言與機械語言仍然相當接近, 它改
用一些特別的字元來代替一大串 0與 1 所
組成的機械語
言碼, 增加了
程式的可讀
性。
7-4
組合語言
• 由於以機械語言與組合語言來編寫程式需
要非常瞭解電腦的內部構造, 並且直接控制
電腦硬體, 因此我們稱此兩者為低階語言。
• 對於缺乏電腦專業知識背景的人, 這兩種語
言學習起來還是滿困難的, 所以才有以下高
階語言的發展。
7-5
高階語言
• 高階語言是比較接近人類使用的電腦程式
語言。這類電腦語言採用類似英文的字彙
及數學運算式的語法來敘述人類要電腦做
的事, 因此用高階語言撰寫程式比較輕鬆,
而程式也比較容易閱讀。
• 目前較常用的高階語言有：Visual Basic、
Delphi、Java...等；早期流行的有BASIC、
FORTRAN、COBOL、C...等。
7-6
Chapter 7
Low-Level Programming
Languages
Computer Operations
• A computer is a programmable electronic
device that can store, retrieve, and
process data
• Data and instructions to manipulate the
data are logically the same and can be
stored in the same place
• Store, retrieve, and process are actions
that the computer can perform on data
Machine Language
• Machine language The instructions built
into the hardware of a particular computer
• Initially, humans had no choice but to write
programs in machine language because
other programming languages had not yet
been invented
Machine Language
• Every processor type has its own set
of specific machine instructions
• The relationship between the processor
and the instructions it can carry out is
completely integrated
• Each machine-language instruction does
only one very low-level task
Pep/7: A Virtual Computer
• Virtual computer A hypothetical machine
designed to contain the important features
of real computers that we want
to illustrate
• Pep/7
– designed by Stanley Warford
– has 32 machine-language instructions
• We are only going to examine a few
of these instructions
Features in Pep/7
• The memory unit is made up of 4,096 bytes
• Pep/7 Registers/Status Bits Covered
– The program counter (PC) (contains the address
of the next instruction to be executed)
– The instruction register (IR)
(contains a copy of the instruction being executed)
– The accumulator (A register)
– Status bit N (1 if A register is negative; 0 otherwise)
– Status bit Z (1 if the A register is 0; and 0 otherwise)
Features in Pep/7
Program Counter
應為16bits，雖然
前4碼都為0
Figure 7.1 Pep/7’s architecture
Instruction Format
• There are two parts to an instruction
– The 8-bit instruction specifier
– And optionally, the 16-bit operand specifier
Figure 7.2 The Pep/7 instruction format
Instruction Format
• The instruction specifier is made up of
several sections
– The operation code
– The register specifier
– The addressing-mode specifier
Instruction Format
• The operation code specifies which
instruction is to be carried out
• The 1-bit register specifier is 0 if register A
(the accumulator) is involved, which is the
case in this chapter.
• The 2-bit addressing-mode specifier says
how to interpret the operand part of the
instruction
Instruction Format
Figure 7.3 Difference between immediate-mode and direct-mode addressing
Some Sample Instructions
Figure 7.3 Subset of Pep/7 instructions
A Program Example
• Let’s write "Hello" on the screen
Page 200
Pep/7 Simulator
• A program that behaves just like the Pep/7
virtual machine behaves
• To run a program, we enter the hexadecimal
code, byte by byte with blanks between each
Page 202
Assembly Language
• Assembly languages A language that
uses mnemonic codes to represent
machine-language instructions
– The programmer uses these alphanumeric
codes in place of binary digits
– A program called an assembler reads each
of the instructions in mnemonic form and
translates it into the machine-language
equivalent
Pep/7 Assembly Language
Figure 7.5 Assembly Process
Pr0502
CHARI h#000D,d ;Input first character
CHARI h#000E,d ;Input second character
CHARO h#000E,d ;Output second character
CHARO h#000D,d ;Output first character
STOP
.BLOCK d#1
;Storage for first char
.BLOCK d#1
;Storage for second char
.END
Assembler Listing
Object
Addr
code
0000
D9000D CHARI h#000D,d
;Input first character
0003
D9000E CHARI h#000E,d
;Input second character
0006
E1000E CHARO h#000E,d
;Output second character
0009
E1000D CHARO h#000D,d
;Output first character
000C
00
STOP
000D
00
.BLOCK d#1
;Storage for first char
000E
00
.BLOCK d#1
;Storage for second char
000F
Mnemon
.END
Operand
Comment
Pr0502-modified
CHARI c1,d ;Input first character
CHARI c2,d ;Input second character
CHARO c2,d ;Output second character
CHARO c1,d ;Output first character
STOP
c1:
.BLOCK d#1
;Storage for first char
c2:
.BLOCK d#1
;Storage for second char
.END
Assembler Listing
Object
Addr
code
Symbol Mnemon
0000
D9000D
CHARI c1,d
;Input first character
0003
D9000E
CHARI c2,d
;Input second character
0006
E1000E
CHARO c2,d
;Output second character
0009
E1000D
CHARO c1,d
;Output first character
000C
00
STOP
000D
00
c1:
.BLOCK d#1
;Storage for first char
000E
00
c2:
.BLOCK d#1
;Storage for second char
000F
.END
Symbol Value
Symbol Value
c1
c2
000D
000E
Operand
Comment
A New Program
Our Completed Program
Page 217 Pseudo Code7-22
Object
Addr
code
0000
70000B
0003
0000
0005
Symbol Mnemon Operand
BR
Main
sum:
.WORD
d#0
0000
num1:
.BLOCK
d#2
0007
0000
num2:
.BLOCK
d#2
0009
0000
num3:
.BLOCK
d#2
000B
090003 Main:
LOADA
sum,d
000E
E90005
DECI
num1,d
0011
190005
ADDA
num1,d
0014
E90007
DECI
num2,d
0017
190007
ADDA
num2,d
001A
E90009
DECI
num3,d
001D
190009
ADDA
num3,d
0020
110003
STOREA sum,d
0023
F10003
DECO
0026
00
STOP
0027
.END
sum,d
Symbol
Main
num2
Sum
Value
000B
0007
0003
Symbol Value
num1
0005
num3
0009
Status Bits
Status bits allow a program to make a choice.
BRLT
Set the PC to the operand, if N is 1
(A register is less than zero)
BREQ
Set the PC to operand, if Z is 1
(A register is equal to zero)