# PC/AT

PC-compatible machines need no introduction. They are one of the
most popular machines of all time. Collapse OS has a 8086
assembler and has boot code allowing it to run on a
PC/AT-compatible machine, using BIOS interrupts in real mode.
Collapse OS always runs in real mode.

In this recipe, we will compile Collapse OS and write it to a
USB drive that is bootable on a modern PC-compatible machine.

# Gathering parts

* A modern PC-compatible machine that can boot from a USB drive.
* A USB drive

# Build the binary

Running "make" in /arch/8086/pcat will yield:

* mbr.bin: a 512 byte binary that goes at the beginning of the
           disk
* os.bin: 8086 Collapse OS binary
* disk.bin: a concatenation of the above, with "blkfs" appended
            to it starting at 0x2000.

disk.bin is what goes on the USB drive.

This binary has BLK and AT-XY support, which means you have disk
I/Os and can run VE.

# Emulation

You can run the built binary in Collapse OS' 8086 emulator using
"make emul".

The 8086 emulator is barbone. If you prefer to use it on a more
realistic setting, use QEMU. The command is:

    qemu-system-i386 -drive file=disk.bin,if=floppy,format=raw

# Running on a modern PC

First, copy disk.bin onto your USB drive. For example, on an
OpenBSD machine, it could look like:

    doas dd if=disk.bin of=/dev/sd1c

Your USB drive is now BIOS-bootable. Boot your computer and
enter your BIOS setup to make sure that "legacy boot" (non-EFI
boot, that is, BIOS boot) is enabled. Configure your boot device
priority to ensure that the USB drive has a chance to boot.

Reboot, you have Collapse OS. Boot is of course instantaneous
(we're not used to this with modern software...).