n301/cs16basicmachinelang n301.tplt Stored Program The most fundamental characteristic of the Von Neumann architecture A sequence of machine language instructions stored as binary values in memory Task of the Control Unit to
-   fetch from memory the next instruction to be executed
-   decode it
-   execute it Machine Language Instructions Instructions that can be decoded and executed by the control unit of a computer Operation code (op code) is a unique unsigned-integer code assigned to each machine language operation Address field(s) - the memory addresses of the values on which this operation will work Format Typical Machine Language Format Instruction Format Assume the op code for ADD is a decimal 9 Cells X and Y correspond to addresses 99 and 100 (decimal)

op code	address 1	address 2
8 (bits)	16 (bits)	16 (bits)

Putting it all together

op code	address 1	address 2
00001001	0000000001100011	0000000001100100

Instruction Set The set of all operations that can be executed by a processor RISC - Reduced Instruction Set Computer - newer processors make instruction sets as small and as simple as possible Instruction Groups 4 basic groups
-   data transfer
-   arithmetic
-   compare
-   branch Data transfer These operations that move information between or within the different components of the computer All data transfer instructions follow the nondestructive fetch/destructive store principle Contents of the source cell are never destroyed, only copied. Contents of the destination cell are overwritten and its previous contents are lost Example Data transfer

Operation	Meaning
LOAD X	Load register R with the contents of memory cell X
STORE X	Store the contents of register R into memory cell X
MOVE X, Y	Copy the contents of memory cell X into memory cell Y

Arithmetic These are operations that caue the arithmetic/logic unit to perform computation. Typically indluce arithmetic operations like + , - , * , / as well as logic operations such as AND, OR and NOT Arithmetic Example

Operation	Meaning
ADD X, Y, Z	Add the contents of memory cell X to the contents of memory cell Y and put the result in memory cell Z (Three-address ihnstruction)
ADD X, Y	Add the contents of memory cell X to the contents of memory cell Y. Put the result into memory cell Y (two-address instruction)
ADD X	Add the contents of memory cell X to the contents of register R. Put the result into register R (one-address instruction)

Compare These operations compare two values and set an indicator on the basis of the results of the compare Most Von Neumann machines have a special set of bits inside the processor called condition codes and these bits are set by the compare operations Compare Example

Operation	Meaning
COMPARE X, Y	Compare the contents of memory cell X to the contents of memory cell Y and set the condition codes accordingly

Branch The normal mode of operation of a Von Neumann machine is sequential The branch instructions alter the normal sequential flow of control - based typically on the current settings of the condition codes Branch is almost always proceded by either a compare instruction or some other instruction that sets the condition codes Branch Example

	Condition	How the condition codes are set
CON (X) > CON (Y)	GT = 1	EQ = 0	LT = 0
CON (X) = CON (Y)	GT = 0	EQ = 1	LT = 0
CON (X) < CON (Y)	GT = 0	EQ = 0	LT = 1

Operation	Meaning
JUMP X	Take the next instruction unconditionally from memory cell X
JUMPGT X	If the GT indicator is a 1, take the next instruction from memory cell X. Otherwise, take the next instruction from the next sequential location
HALT	Stop program execution. Don't go on to the next instruction

Fetch/Execute The task of the control unit is to fetch and execute instructions similar to what was shown in the previous slides The control unit relies on two special registers to accomplish this:
-   Program Counter (PC)
-   Instruction Register (IR)
-   and on the Instruction Decoder Circuit Sequence The PC holds the address of the next instruction to be executed The IR holds a copy of the instruction fetched from memory (holding both the op code and the addresses) To determine what the instuction is, the op code portion must be decoded using the instruction decoder Assembly Language Developed by individuals programming on the first-generation computers - designed for people as well as computers Today, assembly language viewed as low-level programming languages - which means they are closely related to the machine language High-level languages Because they are more user-oriented, they are not machine-specific, the use both natural language and mathemetical notation in design
-   BASIC
-   Pascal
-   C++
-   Java The Continuum of Programming Languages Source Program User written program Translated into a machine language program - object program - carried out by the assembler (compiler) Advantages of assembler Use of symbolic operation codes rather than numeric ones Use of symbolic memory addresses rather than numeric ones Psuedo-operations that provide useful user-oriented services such as data generation Translation/Load/Execute Format

label:

op code mnemonic

address field

--comment

Label Attached to any instruction or piece of data in the program Becomes a permanent identification for this instruction or data, regardless of where it appears in the program or where it may be moved in memory Name followed by a colon placed at the beginning of an instruction
-   BEGIN:     LOAD X Op code mnemonic Allows the programmer to refer to op codes using the symbolic name rather than by a number
-   LOAD, ADD, STORE Address field Lets the programmer use symbolic addresses in addition to numeric addresses
-   JUMP 17 Comment Helpful explanation added to the instuction by a programmer and intended for someone reading the program Ignored during translation and execution Instruction Set