I understand how the code is compiled to assembly, and that assembly is a 1:1 replacement with binary codes. Can somebody help me understand how binary is connected to the hardware? How is the binary physically read and run? How does an if statement work in the hardware?
From google searches I'm thinking that maybe my question title should be "how is binary data put on a line of a bus" but I wasn't sure.
Thanks.
I think this is actually a fun question. I would say "here's how to build a computer in a few easy steps".
Start with some simple logic circuits, such as AND, OR, NOT, and a flip-flop. A flip-flop is a pair of transistors arranged so that if one is ON, the other is OFF, or vice-versa. That way it can "remember" one bit of information, so you can think of it as a storing a single binary digit. Some input lines can put it in one state or the other, and thus "write" to it.
You can store a bigger number by having a bunch of flip-flops, and call it a "register". For example, if you have four flip-flops in a register, there are 16 possible combinations, so you can think of it as holding a number from 0 to 15.
Skipping ahead a little bit, you can buy a "memory chip". What that is is a good number of registers, like say 16 of them. It has 4 wires coming in (the "address" wires), and it has 4 wires coming out (the "data" wires). So a number from 0 to 15 can come in as an address, and that selects one of the 16 registers, whose value is presented on the output data wires (thus "reading" it). Another few wires can cause data to come IN on the data wires to cause numbers to be put into ("written") the register.
Now suppose you have an external 4-bit register (call it R), and a bit of circuitry, so that it
Depending on the numbers that have been pre-loaded into the memory, you can see that this thing will cycle around through a series of numeric addresses, because the number at each address determines what the next address will be.
Now, you can embellish this thing in a lot of ways. You can expand the memory to have 5-bits of address (32 registers). Then if one of the address lines is connected to the outside world, it will do different things depending on the outside world. That's a "finite-state-machine".
You can replace the R register with a simple counter, and call it a "program counter". You can take the data coming out of the memory and call it an "instruction", and use some of its bits to read other memory addresses and load a set of arithmetic registers. You can use some to control whether the R register simply increments, or maybe gets a new address stored in it. That's called "jumping".
Of course, this is a very simple computer, but that's roughly how they started out.
A complete answer to your question would encompass a book, and a fairly thick one at that.
When you say "code" I'm assuming you're referring to a high level compiled language, like C++. Usually, a compiler translates this code into machine language, or binary as you state in your question. We'll neatly avoid all discussion of managed vs. unmanaged code, p-code, etc. That is, we're just talking about compilers that target specific processors/operating systems. Java, for example, compiles into a pseudo-code called bytecode. We're also going to avoid the whole matter of link editing, or linking, which is how multiple source modules get compiled into machine language then bound together into a single executable program.
Okay, now that we've covered most of what we're not going to cover, here's what usually happens. And by "usually", I mean most compiled languages in a DOS, Linux or Windows environment. The source code is translated into machine language, which is written out to an executable file. This executable file contains, more or less, an image of what the program should look like in memory. When you tell the operating system to run your program, the OS's equivalent of a "Load and Go" executes. What that means is that the memory image in the executable file is loaded into memory, then the operating system does a machine language JUMP to the first instruction in the program. The CPU then blindly follows the instructions from thereon, until an EXIT is encountered.
This whole JUMP... EXIT nastiness is a drastic oversimplification for modern OS's. As you can imagine, if the CPU were to follow, with blind obedience, the instructions in a program that's gone astray, the computer would crash... or worse. Such was the fate of many an errant program in the early days, and a prime contributor to many a BSOD.
This is a huge, very complicated topic. The best textbook I've seen on the subject is Patterson/Hennesy's "Computer Organization and Design", which has many editions.
Other than suggesting you read it, I wouldn't dare try to cram a semester-long class into a 500-character answer box.
There is a Book named code which goes over that as well as any computer organization text. Thogh the answers here are all good.
I dont see this as huge and complicated, the closer to the hardware the simpler it gets.
Write a disassembler, thats how the hardware does it. Most processors include the opcodes or instruction set in the same manual as the assembler language.
Look at the opcode for say an add instruction using registers, a few of the bits determine the source register, a few bits for destination register a few bits say that this is an add instruction. Let's say this instruction set you are looking at uses only two registers for a register based add. There is some logic, an adder, that can add two items the size of registers and output a result and a carry bit. Registers are stored on chip in memory bits sometimes called flip flops. So when an add is decoded the input registers are tied to the add logic using electronic switches. These days this happens at the beginning of the clock cycle, by the end of the clock cycle the adder has a result and the output is routed to the bits for the destination register and the answer is captured. Normally an add will modify the flags in the flag register. When the result is too big to be stored in the register (think about what happens when you add the decimal numbers 9 and 1 you get a 0 carry the 1 right?). There is some logic that looks at the output of the adder and compares the bits with the value zero that sets or clears the z flag in the flag register. Another flag bit is the sign bit or n bit for negative, that is the most significant bit of the answer. This is all done in parallel.
Then say your next instruction is jump if zero (jump if equal), the logic looks at the z flag. If set then the next instruction fetched is based on bits in the instruction that are added to the program counter through the same or another adder. Or perhaps the bits in the instruction point to an address in memory that hold the new value for the program counter. Or maybe the condition is false, then the program counter is still run through an adder but what is added to it is the size of the instruction so that it fetches the next instruction.
The stretch from a disassembler to a simulator is not a long one. You make variables for each of the registers, decode the instructions, execute the instructions, continue. Memory is an array you read from or write to. The disassembler is your decode step. The simulator performs the same steps as the hardware, the hardware just does it in parallel using different programming tricks and different programming languages.
Depending on how implemented your disassembler might start at the beginning of the program and disassemble to the end, your simulator would start at the beginning but follow the code execution which is not necessarily beginning to end.
Old game console simulators like MAME have processor simulators that you can look at. Unfortunately, esp with MAME, the code is designed for execution speed not readability and most are completely unreadable. There are some readable simulators out there if you look though.
A friend pointed me at this book http://www1.idc.ac.il/tecs/ which I would like to read, but have not yet. Perhaps it is just the book you are looking for.
Sure hardware has evolved from trivial state machines that take many clocks to fetch, decode, and execute serially. My guess is that if you just understood the classic fetch, decode and execute that is enough for this question. Then you may have other more specific questions, or perhaps I misunderstood the question and you really wanted to understand the memory bus and not the decoder.
(Vastly simplified)
The binary (say a string of binary from a line of machine code/asm) is loaded into memory from say disk. Then an instruction is sent by the processor logic to memory controller to load the contents of the memory into a processor local resister. It then gets interpreted as an instruction to do by the processor.
I learned this level of stuff by doing microcoding at college.
In reality there are many more steps that could occur, depending on the processor complexity and power. The processor is made up of various parts (ALU, registers etc) and they cooperate in getting instructions, data and processing. If you are interested in this level of understand and I commend you for asking the question, Id say get a book on computer architecture. I used Structure Computer Organisation by Tanenbaum at college.
When we look at Binary or a programming language we are looking at symbols and words that a human can understand which describe essentially the electrical activity of an electronic system. Ultimately, speed of energy, amount of energy, loss of energy, creation of heat and light, and the CHANGE that the substance which the energy is flowing through undergoes as a result of being energized as well as how that effects the activity of the energy as it flows is understood by a human and manipulated or harnessed with a piece of electronics. One factor in understanding the complex system which is a computer or other device, is knowing what the electricity that's flowing inside of it is doing. To know this we have described one of the electricity's behaviors using the numbers 1 and 0. You must know what a physical element is doing and be able to give a standard description of that in order to keep track of what happens as you change the factors that you think contribute to the control of the element/Substance/Subatomic Particle, the numbers help you to rationalize what happening with particles that you are otherwise unable to see.
So Binary is a visual representation which uses numbers to describe the activity of electricity in piece of hardware. Either their is a flow of charge or their is no flow of charge. The charge is converted to D/C from AC and the activity of the charged particles is stabilized by the clock, the speed of the energy flowing through the circuit, the resistance (which results in a loss of energy and the creation of heat, and the amount of time before a piece of a circuit is de-energized all comes together allowing us to understand and use our understanding of these phenomenon to come up with a unit of measure to standardize the amount of energy flowing per a unit of time. Which further assists us as we try to harness the energy. The rest of the computers components are like a really advanced system of Transistors, capacitors, and resistors that manipulate the charge that flows to and through those components, the components trap the charge temporarily or slow it down until the component reaches a certain energy level and then an arc flash occurs causing the stored energy to energize another component or area of the system with a specific (highly controlled) amount of energy. each digit of binary is a representation of a bit, a bit is an explanation of electrical behavior in a certain electrical component(known from here forward as signal), a byte is 8 pieces of electrical signal. It is generally agreed that you need at least 8 bits of electrical signal in order for the computer to make orderly and practical use of the electricity flowing into it.
In other words electricity is just reduced and energy stabilized in its behavior and then split up and directed through a series of components which need electricity to operate and then the newly energized components do a thing that a human desires for it to do.
This seems like a 'how do computers work' question but you're not likely to be satisfied by the answers you get from Google.
The details of how binary is 'connected' to the hardware is really the subject of computer design at a basic level. Some knowledge of hardware design is very useful for programmers, but not strictly necessary.
The answers, so far, are 'Vastly simplified' because, well, they have to be.
In general the explanations will only make sense to someone who understands the subject. I don't think there's much we can do about that.
EDIT: perhaps the best we can say is that the bits in binary numbers (the zeroes and ones) correspond directly to the circuits in the hardware because circuits can be configured to 'store' a bit value, and those bits can be manipulated by the circuitry.
That is, they can be stored, counted, added (and all the other basic arithmetic functions), output and input in groups forming whole numbers.
Briefly, the bits in the binary numbers correspond to the transistors in the hardware. This, as the others have said, is an enormous simplification. It frequently takes more than one transistor to handle a bit, but that depends on the cleverness of the hardware designer.
SW is not just the SW language it is written in, as say jotted down on a piece of paper. SW takes on physical form as well. At some point the software at the conceptual level crosses over into software at the physical level, and that occurs when a programmer starts typing code into a keyboard in whatever SW language he/she is working. From the moment a keystroke is tapped, it's electrons all the way down...this is the point then, where the interface occurs, in that from the moment a keyboard is tapped the whole business becomes the manipulation of electrons -- as complex, sophisticated and ingenious an endeavor as that may be. Thinking in terms of binary 0's and 1's is just a metaphor for high & low voltage, already a physical manifestation beyond the keystroke. When you type the letter I as the first letter of IF...THEN into a keyboard, the voltages corresponding to 01001001 are placed in the first 8 slots of the registry, through electrical impulses prompted by your physically tapping the I key. From here on out it's electronics.
Essentially, and very broadly speaking, the instructions end up in memory, and the program counter (PC) register holds the address of the first instruction.
The processor supports instructions that load can move data to/from memory to registers. The processor moves data into the instruction register, and that instruction gets executed at the next clock tick.
I'm not qualified to explain the electrical engineering behind that, but you could probably look it up.
Of course, this all fairly simplified, as there is a huge amount of parallelism going on in modern processors, and I don't even pretend to grok that in any meaningful way.
You may find this link of interest: http://www.eskimo.com/~ddf/Theory/Micro_Fund/Commands/AssyLnge.html
In a short explanation, typically the application will be loaded to some area in memory and the processor will be given the start address, for the main or starting part.
A processor can do two things, it can manipulate data and move data, so it will use special parts on the microprocessor called registers in order to store values or location that it will need.
So, for an if statement, there is a command for whether two values are equal and where to go if they are not (branch if not equal). There will be some others, such as branch of equal or branch of greater than.
These will be in the assembly.
The commands are put into registers, with the appropriate arguments and it will be calculated on the next clock cycle.
This is very simplified, but to a large extent, what you see in assembly is what the microprocessor will see.
To go into more detail would probably be out of scope for StackOverflow.
In a very simplified way the computer can be represented as an infinite loop (implemented in hardware) and abilities to do simple arithmetic operations (also implemented in hardware). In the loop it does the following:
And that's it. Also there are control commands which can change the PC and which are used for "if ... then ... else" statement.
This question is very complicated, I have 2 degrees in exactly that and I've still only scratched the surface.
If you want an intro to how that all works together, MIT has some free classes available you can look over online. This one is probably the best one to get you started.
Every thing you write on a text editor, first, it is stored in a memory (electric signals generated from the keybord) no matter in which code (ascii,...). From memory, those signals are fed into the computer monitor and you can see the source code you are typing. Then, you run your compiler (or assembler) that reads the source code in memory (electric signals) and converts it to machine code, storing those transformed electric signals in another region of the memory (electric signals again). When the cpu reads the machine code, what it sees are electric signals. There is no logic levels, so there is no need to convert logic level in voltage level.
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