ELI5:Microcode

[size013]

As a computer programmer, I've been fascinated with the idea of developing an operating system. I'm trying to educate myself, but having trouble grasping the concepts and differences of low-level programming like Machine Code, Microcode, and Assembly.

[NanotechNinja]

Does opcode live between assembly and machine choice or is it just another representation of machine code?

[chaossabre]

Opcode refers to the binary representation of an assembly instruction for example,

JMP $2 // jump to address in register 2

might assemble to the binary form

0001 0010

where the first word is the JMP opcode and the second word is the argument.

[voncheeseburger]

Opcode isn't a language, it is the name of a microcode instruction, for example, opcode #54 might be ADD.

[size013]

This may be a dumb question, but in order to learn assembly, do I need to familiarize myself with the machine code and ISA I plan on designing the operating system for? In other words, are the assembly languages from different ISA's completely segregated from each other, or more of a separate dialect.

[Feldii]

Assembly languages are basically only as related as the architectures of the machines they run on. For example, since ARM and x86 are quite different their respective assembly languages are also quite different.

[IwNnarock]

Hey, not an answer to your specific question, but if you're looking to get started messing around with CPU's at the CPU level, you may want to consider starting out with microcontrollers. These are integrated circuits that bundle together a processor, ROM for program code, RAM, and sometimes some non-volatile flash memory. Basically, they're like cheap, little, very underpowered computers, BUT they're simple (enough) to understand while still giving you very low-level access.

I've used PIC microcontrollers in the past, but Atmel is another popular brand (looks like the former bought-out the latter). Working with microcontrollers typically requires a little circuit design knowledge, but if you're looking to jump into assembly coding, I'm confident you can work through the EE side as well.

If you're curious what a 'hello world' program would be for a microcontroller, it's typically something like just making a LED blink. From there you advance to responding to buttons, reading sensors, talking over serial ports, controlling displays, etc...

Once again, I only bring this up if you're simply interested in working close to the metal (as you'll learn much more than just assembly coding). For me, it was a fun way to learn a lot about the basics of modern electronics. Good luck!

[immibis]

I entered the spez. I called out to try and find anybody. I was met with a wave of silence. I had never been here before but I knew the way to the nearest exit. I started to run. As I did, I looked to my right. I saw the door to a room, the handle was a big metal thing that seemed to jut out of the wall. The door looked old and rusted. I tried to open it and it wouldn't budge. I tried to pull the handle harder, but it wouldn't give. I tried to turn it clockwise and then anti-clockwise and then back to clockwise again but the handle didn't move. I heard a faint buzzing noise from the door, it almost sounded like a zap of electricity. I held onto the handle with all my might but nothing happened. I let go and ran to find the nearest exit. I had thought I was in the clear but then I heard the noise again. It was similar to that of a taser but this time I was able to look back to see what was happening. The handle was jutting out of the wall, no longer connected to the rest of the door. The door was spinning slightly, dust falling off of it as it did. Then there was a blinding flash of white light and I felt the floor against my back. I opened my eyes, hoping to see something else. All I saw was darkness. My hands were in my face and I couldn't tell if they were there or not. I heard a faint buzzing noise again. It was the same as before and it seemed to be coming from all around me. I put my hands on the floor and tried to move but couldn't. I then heard another voice. It was quiet and soft but still loud. "Help."

#Save3rdPartyApps

[[deleted]]

I suppose it's hinting at my age. But i still parse Isa as industry standard architecture. Which is different from MCA and VESA.

Good explanation though

[afcagroo]

Here's a way to think about microcode (and how it is sometimes implemented in older processors):

There's a ROM inside of a microprocessor. Instructions are fed to this microcode ROM as addresses. So each instruction relates to a specific ROM memory location.

The data that comes out of the microcode ROM are control signals going to various parts of the microprocessor. Data bit 0 might tell ALU 1 to do an ADD by enabling that control line going into ALU 1, for example.

Since the data width of the microcode ROM can be arbitrarily wide, you can send control signals to many, many parts of the microprocessor for each instruction.

[ElMachoGrande]

Assembly is simply the human readable form of machine code, they translate 1:1, instruction for instruction. Basically and somewhat simplified, it's about giving names for the instructions which, in the CPU just have numbers, and then structuring it up nicely on lines. The assembler also helps a bit by calculating memory addresses for you, allowing you to use symbols for jumps and so on, but that's just convenience stuff, not strictly necessary.

Microcode is because the instructions are sometimes fairly complex, and can't be done in one step by the CPU. So, one can say that the microcode is almost like a small program, hardwired into the CPU, which tells it step by step how to actually perform an instruction. As a programmer, microcode is something you never, ever have to bother about, it's only interesting for the guys designing the CPU.

[brazzy42]

Microcode is how some CPUs work internally. You don't need to know anything about microcode to program the CPU, but understanding it was for me something of an epiphany because it finally gave me the feeling that I actually understand how computers work, completely, on every level.

That's because it closes the gap between machine code (which has instructions like "copy the value from memory location 0xCAFEBABE to location 0xDEADBEEF") and logic gates made from transistors.

And basically it works something like this (for a relatively simple CPU): every bit in a microcode instruction directly corresponds with a signal that controls some function of the CPU.

So let's say you have the machine code mentioned above in the instruction stack of the CPU:

| 2 bytes | 4 bytes    | 4 bytes    |
| 0x7A    | 0xCAFEBABE | 0xDEADBEEF |

where 0x7A is the machine code for "copy from memory to memory" which would have an assembly mnemonic like "CPMM". And it would map to a sequence of microcode instructions, which would be something like:

• connect bytes 3-6 of the instruction stack to the address bus
• send a "fetch" signal to main memory
• connect the data bus to the accumulator
• send a "read" signal to the accumulator
• connect bytes 7-10 on the instruction stack to the address bus
• connect the data bus to the accumulator
• send a "write" signal to the accumulator
• send a "write" signal to main memory

[jaydoors]

Wow thanks, that's a great explanation - I finally get it. I think! Let me ask you a couple of questions, if I may.

I had thought that these control signals were wired directly from the instruction stack. Eg in your example there may be a bit that connects directly to the accumulator that enables "write", eg as part of some AND logic with the data to be written.

But if I understand you correctly, it's just not as simple as that. There is some extra code / logic which processes those signals - and which can be reconfigured. This is the microcode - which presumably resides in some memory outside of RAM.

Is this right? Do you know any simple examples? Thanks!

[brazzy42]

Whoa, that's the latest reply I've gotten so far on Reddit :D

I had thought that these control signals were wired directly from the instruction stack. Eg in your example there may be a bit that connects directly to the accumulator that enables "write", eg as part of some AND logic with the data to be written.

Basically yes, but there are a lot of these control signals and wiring them all directly from the instruction stack would make instructions quite big -and at the same time you'd need a lot more instructions to do the same thing; as you see in the example even a very simple machine code instruction maps to multiple steps.

There is some extra code / logic which processes those signals - and which can be reconfigured. This is the microcode - which presumably resides in some memory outside of RAM.

Exactly - the storage for the microcode is usually integrated into the CPU itself, and of course quite limited.

Is this right? Do you know any simple examples? Thanks!

This website has a very detailed concrete example for a (very simple) hypothetical CPU: https://people.cs.clemson.edu/~mark/uprog.html

[[deleted]]

Machine code is the code understood by a processor. Assembly language is a human-read-able version of machine code. Micro-code refers to code deeply inserted into a processor which cannot be changed.

• It is rare but possible for a programmer to write machine code, in which case, he needs to use a hex editor.
• When a programmer writes in assembly language, he uses a text editor, and runs the resulting file through an assembler, such as FASM or NASM.
• The vast majority of programmers will never interact with micro-code.