Ask HN: How does code get translated to voltage inside a CPU transistor?

Text in my text editor gets converted to 1's and 0's which then get converted to electric signals inside transistors in my iPad's CPU.

What's the mechanism by which 1's and 0's turn into different voltages? Can someone point me to a resource (preferably a video) that explains this?

(How is mere text able to control what happens inside an inanimate machine?)

(In other words, what happens at the software|hardware boundary?)

In brief, Code is stored in memory as a series of 1's and 0's.

Now the memory is stored as different voltage levels in transistors - 5 volts for a 1 and 0 Volts for a 0 for argument sake (but the voltage to logic value is arbitary).

Now the CPU executes the code by walking along different pieces of memory and executing different instructions depending on the value of those pieces of memory.

There's a lot more. For a start Google 'Introductory Digital Electronics' or 'Introductory Computer Architecture' and this might get you started.

I'd draw the boundary this way - hardware talks about how physical components will affect behavior of voltage levels, software talks about how one voltage levels will affect behavior of other voltage levels.

There's a think called a J-K flip flop that is worth having a look at - it's the simplest memory element conceptually and it takes a number of transistors to actually implement (6 I think from memory in TTL - I may be wrong - it was a long time ago).

In a real cpu D-Ram stores the bits (1's and 0's) and it works by storing a charge http://en.wikipedia.org/wiki/Dynamic_random-access_memory

The software / hardware boundary you speak of is a matter of definition and different people would define it differently. For a hardware developer, they would probably regard microcode as software, but an application developer would regard it as part of the hardware.

hth

I have a BS in Computer Engineering and a MS in Electrical Engineering, with a specialty in VLSI and microelectronics.

While in school, I had to design a microprocessor, one transistor at a time. Great experience, by the way.

I will summarize this process from a Top-Down approach, in case you're a CS major. (Note that engineering is Bottom-Up!)

- C/C++ gets converted into Assembly Language.

- Assembly Language gets converted into bytes. (One or more bytes encode each machine instruction and data)

- Each machine instruction takes multiple clock cycles to execute.

- On each clock cycle, a "microcode" (a.k.a. Register Transfer Language) moves data around. (The order in which data are moved from one register to another is controlled by a Finite State Machine.)

- Each finite state machine is composed of sequential circuits and combinational circuits: multiplexers, decoders, etc.

- Each sequential circuit is composed of some type of memory element : flip-flops, latches, etc. (Sequential circuits can be static, quasi-static, or dynamic. In all cases, they are composed of combinational logic with feed-back.)

- Combinational logic is composed of CMOS (MOSFET) transistors. In the past, we used BJTs... and they're actually making a comeback.

- Transistors are arranged in such a way as to perform logic equations: Inverter, NAND, NOR, etc. (Look up a CMOS Inverter to know what I'm talking about.)

Pretty easy, right!?!

I want to encourage you to look at the following references, in order.

(1) CMOS Digital Integrated Circuits by Kang and Leblebici --the best digital VLSI book ever (2) Digital Logic Circuit Analysis & Design by Nelson et al. --tough book on digital logic, but pretty good (3) Computer System Architecture by Morris Mano --works its way up from logic design (K-maps, etc.) to state machines to assembly language

And now, I'm going to through a curve ball at you: Any algorithm you can implement in software, you can also implement in hardware.

How is this possible? Hint: Think of Finite State Machines. And then look into ASIC and FPGA design of digital filters and digital control systems.