Interrupts

Cat systems support CPU interrupts which immediately stop what the CPU is doing and jump the instruction pointer to new code. You may trigger an interrupt with the int instruction manually, interrupts are also triggered by errors and user input.

Interrupts 0x00-0x0F are considered error interrupts and will halt the machine by default if not handled. All other interrupts will by default do nothing.

List

The following interrupts are given meaning by default:

Interrupt ID	Name	Handleable	Description
0x00	Page Fault	true	Memory could not be accessed (out of bounds, or forbidden). The faulting address is pushed onto the stack as a 32 bit pointer.
0x01	Invalid Instruction	true	Instruction opcode or arguments were invalid
0x02	Divide by zero	true	Tried to divide by zero
0x03	Protection Fault	true	A privileged opcode (e.g. int, iret, in, out, setit) was executed from User mode
0x04 - 0x0F	RESERVED		Reserved for future CPU exceptions
0x10	Syscall	true	Raised by the syscall instruction. The only way for User mode to enter the kernel directly. The argument convention is up to the kernel.
0x70	Receive Input	true	Receives input from an input device (e.g. a keyboard). r1 is the device ID, r2 is the input type (for keyboards: 0 = key down, 1 = key up), and r3 is the value (for keyboards, the keycode — literally 'A' for the A key).
0x71	Hardware Timer Callback	true	Called by a hardware timer when it completes. r1 holds the timer ID.
0x72	Disk Operation Finish	true	Raised by a disk drive when an asynchronous read/write completes. r1 holds the disk ID.
0x73	NIC Notification	true	Raised by a virtual network card when a packet has been received. r1 holds the NIC ID.
0x80	Write to sout	false	Writes the null-terminated string pointed to by r1 to the host console.
0x81	Halt	false	Pause execution permanently
0x82	Shutdown	false	Poweroff
0x83	Reset	false	Restart (ip = 0)
0x85	Get Uptime MSEC	false	Returns the uptime in milliseconds in r0
0x90	DEBUG: Print Number	false	Prints the number in r1 followed by \n. Only available when EnableTestingInterrupts is set on the VM.

As shown above, some interrupts exist as function calls into the system, allowing you to do things like write to the VM's console. You can call them like functions.

The display, disk drives, network cards and similar peripherals are no longer accessed via dedicated interrupts. They are all serial devices now — see the Display and Serial Protocol pages for how to interact with them.

Handling

You may handle interrupts by constructing an interrupt handler table and then setting the it register (via the privileged setit instruction) to a pointer to it. The structure of the table is as follows:

Offset	Field	Size (Bytes)
0x00	Entry Count	1
0x01	Entries	5 per entry

Then each entry is as follows:

Offset	Field	Size (Bytes)
0x00	Interrupt Code	1
0x01	Handler Pointer	4

When a handler is dispatched from User mode the CPU automatically switches sp to ksp, clears Virtual Mode, and pushes a frame containing the previous mode, mbase, mlen, sp, fl and ip. Handlers must return via the iret instruction, which restores that frame; ret is reserved for ordinary call / ret pairs.