INSTRUCTION GROUPS

Monday, April 27, 2009
HISTORY
Intel did release some 4bit processors in the beginning but the first meaningful processor was 8080, an 8bit processor. The processor became popular due to its simplistic design and versatile architecture . Based on the experience gained from 8080, an advanced version was released as 8085. The processor became widely popular in the engineering community again due to its simple and logical nature.

Intel introduced the first 16bit processor named 8088 at a time when the concept of personal computer was evolving. With a maximum memory of 64K on the 8085, the 8088 allowed a whole mega byte. IBM embedded this processor in their personal computer. The first machines ran at 4.43 MHz; a blazing speed at that time. This was the right thing at the right moment. No one expected this to become the biggest success of computing history. IBM PC XT became so popular and successful due to its open architecture and easily available information.

The success was unexpected for the developers themselves. As when Intel introduced the processor it contained a timer tick count which was valid for five years only. They never anticipated the architecture to stay around for more than five years but the history took a turn and the architecture is there at every desk even after 25 years and the tick is to be specially handled every now and then.

Labels: Introduction to Assembly Language

Posted by atezaz at 1:26 PM 0 comments

INTEL IAPX88 ARCHITECTURE
Now we select a specific architecture to discuss these abstract ideas in concrete form. We will be using IBM PC based on Intel architecture because of its wide availability, because of free assemblers and debuggers available for it, and because of its wide use in a variety of domains. However the concepts discussed will be applicable on any other architecture as well; just the mnemonics of the particular language will be different.

Technically iAPX88 stands for “Intel Advanced Processor Extensions 88.” It was a very successful processor also called 8088 and was used in the very first IBM PC machines. Our discussion will revolve around 8088 in the first half of the course while in the second half we will use iAPX386 which is very advanced and powerful processor. 8088 is a 16bit processor with its accumulator and all registers of 16 bits. 386 on the other hand, is a 32bit processor. However it is downward compatible with iAPX88 meaning that all code written for 8088 is valid on the 386. The architecture of a processor means the organization and functionalities of the registers it contains and the instructions that are valid on the processor. We will discuss the register architecture of 8088 in detail below while its instructions are discussed in the rest of the book at appropriate places.

Labels: Introduction to Assembly Language

Posted by atezaz at 1:26 PM 0 comments

INSTRUCTION GROUPS
Usual opcodes in every processor exist for moving data, arithmetic and logical manipulations etc. However their mnemonics vary depending on the will of the manufacturer. Some manufacturers name the mnemonics for data movement instructions as “move,” some call it “load” and “store” and still other names are present. But the basic set of instructions is similar in every processor. A grouping of these instructions makes learning a new processor quick and easy. Just the group an instruction belongs tells a lot about the instruction.

Data Movement Instructions

These instructions are used to move data from one place to another. These places can be registers, memory, or even inside peripheral devices. Some examples are:

mov ax, bx lda 1234

Arithmetic and Logic Instructions

Arithmetic instructions like addition, subtraction, multiplication, division and Logical instructions like logical and, logical or, logical xor, or complement are part of this group. Some examples are:


and ax, 1234
add bx, 0534
add bx, [1200]

The bracketed form is a complex variation meaning to add the data placed at address 1200. Addressing data in memory is a detailed topic and is discussed in the next chapter.

Program Control Instructions

The instruction pointer points to the next instruction and instructions run one after the other with the help of this register. We can say that the instructions are tied with one another. In some situations we don’t want to follow this implied path and want to order the processor to break its flow if some condition becomes true instead of the spatially placed next instruction. In certain other cases we want the processor to first execute a separate block of code and then come back to resume processing where it left.

These are instructions that control the program execution and flow by playing with the instruction pointer and altering its normal behavior to point to the next instruction. Some examples are:

cmp ax, 0 jne 1234

We are changing the program flow to the instruction at 1234 address if the condition that we checked becomes true.

Special Instructions

Another group called special instructions works like the special service commandos. They allow changing specific processor behaviors and are used to play with it. They are used rarely but are certainly used in any meaningful program. Some examples are:

cli sti
Where cli clears the interrupt flag and sti sets it. Without delving deep into it, consider that the cli instruction instructs the processor to close its ears from the outside world and never listen to what is happening outside, possibly to do some very important task at hand, while sti restores normal behavior. Since these instructions change the processor behavior they are placed in the special instructions group.