Assembly programming can be intimida

Found at: gopher.blog.benjojo.co.uk:70/interactive-x86-bootloader-tutorial

 x86 assembly doesn't have to be scary (interactive)

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 Assembly programming can be intimidating for people who have

never looked into it any deeper than a glance, but giving that

it underpins how the computers we use work it can be helpful

having context in regards to what is actually being run by the

CPU.

IBM PC

 IBM PC

 Photo by: [Drew Sadik]

 x86 PCs are impressive in that they are fully backwards

compatible with the original IBM PC. All the way down to timing

circuits and boot process. This is both great in that almost all

PCs should be able to run software from 1983, but this

backwards compatibility also has a very high cost. The x86 ISA is

inframous in it's edge cases and complexity.

 To boot a modern x86 PC, you need to step through the

history of x86, this is handled by the bootloader in 99% of

cases:

flow of a modern system booting

 flow of a modern system booting

 For the case of our demo's we are going to focus on the

first mode. 16 bit "real mode" is the mode that DOS and other

vintage operating systems run on. This mode offers no system

security that we are used to on our modern systems. When a program

is run it has full access to the system and can alter the

operating system if it wants to. This allowed a creative virus scene

to spring up for DOS (though these viruses were more annoying

than actually malicious)

 This mode also has a very simple model for interacting

with hardware. Because PCs had a wide range of hardware they

outsourced a lot of the code to interact with hardware with software

interrupts (often handled by the BIOS)

 This makes the initial stages of OS development easy, since

you only need to know a handful of software interrupts to get

most of of the I/O work done.

 With that, we can start on our first demo:

 
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 You can play around with the demo above. The assembler

(NASM) and VM is being run in your browser thanks to emscripten

and V86.

 So what is happening here? The `mov` instruction moves data

from one place to another. In this case we are purely setting

registers in the CPU and not touching RAM in the code. The `int`

instruction fires off a software interrupt, in our case the 16th one.

 After that, we invoke `hlt` to stop the CPU from doing

anything else. Since we are done with what we wanted to do.

 But hang on, how did we even get to this stage? What did

the computer do to get our code running just then?

 The BIOS is generally the first thing that gets run on

the system, its job is to setup and test that the computer is

working correctly, It also writes out information about the system

so that operating systems later on can easily detect hardware.

 Assuming the BIOS knows what device it wants to boot off,

it will load the first sector (512 bytes) from that, into a

standardised position of memory, and then hand over control by jumping

to that part of memory

memory loading

 memory loading

 In the demo above, the browser made the 512 byte payload,

padded it into a 1.4MB floppy disk image, and gave it into a

emulator to run.

 So what happens when the program you need to run is more

than 512 bytes?

 Luckily for us the BIOS has us covered with disk I/O as

well with int 13h calls that allow easy access to the drives

in the system.

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 This code loads a segment from the next sector of the

virtual floppy disk, and then jumps to it! The int 13h API is

pretty simple, and gives away some of the low level details of

how storage works (though with the invention of flash memory, a

lot of these values are emulated to retain compatibility)

int13 breakdown

 int13 breakdown

 There are a lot of BIOS provided services out there, some

are more official than others, there is a great reference guide

at http://www.ctyme.com/intr/rb-0608.htm (Though I do warn you

about the 90's style porn banner ads at the bottom of these

pages)

 Hopefully this was a fun intro, You can move onwards and

even switch to C (or Rust!) and find more information about the

underlying hardware of most of the computers you sit down and use

daily on http://wiki.osdev.org/

 If you found this post interesting, you might like the

other things I've done! If not you can always stay up to date

with when I push out a new post by following me on Twitter,

or using my blog's RSS if that is more of your thing.

 Until next time!