Computer Systems – Machine &

Assembly code

Objectives

• Machine Code • Assembly Language• Op-code• Operand• Instruction Set

To investigate and understand…

Machine code

Machine code - simple instructions that are executed directly by the CPU

As we should hopefully already know, computers can only understand binary, 1s and 0s. We are now going to look at the simplest instructions that we can give a computer. This is called machine code

Machine code allows computers to perform the most basic, but essential tasks. For this section we are going to use the Accumulator (you met this register earlier) to store the intermediate results of all our calculations. Amongst others, the following instructions are important for all processors:

LDA - Loads the contents of the memory address or integer into the accumulatorADD - Adds the contents of the memory address or integer to the accumulatorSTO - Stores the contents of the accumulator into the addressed location

Assembly code

Assembly code is the easy to read interpretation of machine code, there is a one to one matching, one line of assembly equals one line of machine code:

Machine code Assembly code

000000110101 Store 53

Let's take a look at a quick coding example using assembly code.

LDA #23 ;loads the number 23 into the accumulatorADD #42 ;adds the number 42 to the contents of the accumulator = 65STO 34 ;save the accumulator result to the memory address 34

Instruction Set

Instruction set - the range of instructions that a CPU can execute

There are many different instructions that we can use in machine code, you have already met three (LDA, ADD, STO), but some processors will be capable of understanding many more. The selection of instructions that a machine can understand is called the instruction set. Below are a list of some other instructions that might be used:

ADD ;add one number to another numberSUB ;subtract one number to another numberINC ;increment a number by 1DEC ;decrement a number by 1MUL ;multiply numbers togetherOR ;boolean algebra functionAND ;boolean algebra functionNOT ;boolean algebra functionXOR ;boolean algebra functionJNZ ;jump to another section of code if a number is not zero (used for loops and ifs)JZ ;jump to another section of code if a number is zero (used for loops and ifs)JMP ;jump to another section of code (used for loops and ifs)

Instruction Set

Let us look at a more complex example of assembly code instructions:

LDA #12 ;loads the number 12 into the accumulatorMUL #2 ;multiplies the accumulator by 2 = 24SUB #6 ;take 6 away from the accumulator = 18JNZ 6 ;if the accumulator <> 0 then goto line 6SUB #5 ;take 5 away from the accumulator (this line isn't executed!)STO 34 ;saves the accumulator result (18) to the memory address 34

Machine InstructionYou'll notice that in general instructions have two main parts:

opcode - instruction nameoperand - data or address

Machine Instruction

Depending on the word size, there will be different numbers of bits available for the opcode and for the operand. There are two different philosophies at play, with some processors choosing to have lots of different instructions and a smaller operand (Intel, AMD) and others choosing to have less instructions and more space for the operand (ARM).

• CISC - Complex Instruction Set Computer - more instructions allowing for complex tasks to be executed, but range and precision of the operand is reduced. Some instruction may be of variable length, for example taking extra words (or bytes) to address full memory addresses, load full data values or just expand the available instructions.

• RISC - Reduced Instruction Set Computer - less instructions allowing for larger and higher precision operands.

Machine Instruction

Addressing modesYou might notice that some instructions use a # and others don't,

# = number[no hash] = address

LOAD #10ADD #12STORE 12

This code loads the number 10 into the accumulator, then adds the number 12, it then stores the result 22 into memory location 12

Machine code and instruction setsThere is no set binary bit pattern for different opcodes in an instruction set. Different processors will use different patterns, but sometimes it might be the case that you are given certain bit patterns that represent different opcodes. You will then be asked to write machine code instructions using them. Below is an example of bit patterns that might represent certain instructions.

Machine code

InstructionAddressing

modeExample

0000 STORE Address STO 12

0001 LOAD Number LDA #12

0010 LOAD Address LDA 12

0100 ADD Number ADD #12

1000 ADD Address ADD 12

1111 HALT None HALT

Machine code and instruction sets

Using the table on the last slide, provide machine code to do the following:

LOAD 12ADD #6

Answer

0010 11000100 0110

Machine code and instruction sets

Using the table on the last slide, provide assembly code for the following machine code:

0001 00000111 0100 00001001 0000 00011110

AnswerLOAD 7ADD 9STORE 30

Explain what the above code does:

Machine code in Hexadecimal

People who work with machine code prefer to display in Hexadecimal, this is because it takes less space than binary. Errors are less likely to occur when dealing with smaller data sets.

Machine code

InstructionAddressing

modeHexadecimal Example

0000 STORE Address 0 STO 12

0001 LOAD Number 1 LDA #12

0010 LOAD Address 2 LDA 12

0100 ADD Number 4 ADD #12

1000 ADD Address 8 ADD 12

1111 HALT None F HALT

Machine code in Hexadecimal

Convert the following machine code into hexadecimal:

0001 001110110100 000010010000 000111101111 00000000

Answer13B49

Key terms…

• Machine Code • Assembly Language• Op-code• Operand• Instruction Set