old-trainOS/README.md

trainOS

Description

An operating system built around the language "trainscript" which is used to execute all user code in a virtual machine.

Current status:

The vm can execute some code and is able to call native code. Also it leaks memory. A lot of memory.

Todo List

• Fix memory leaks -- Validation code leaks memory -- ???
• Improve virtual machine / trainScript -- Add support for pointer types -- Restructre code execution into loop-based execution instead of a recursion-based one -- Add support for feature restriction -- Add JIT compiler with thunkers
• Improve kernel -- Add support for "delegates" (callback + state): `void callback(void *state)

Guidlines

• Calls to die or die_extra should follow the following scheme: ContextName.ErrorName

Kernel Architecture

The kernel is a simple kernel that does not utilize paging. It has simple interrupt management where an interrupt handler can be registerd for each interrupt.

Bootstrapping

1. Setup 16 kB stack space
2. Clear screen and print boot message
3. Initialize physical memory management (page allocator)
4. Initialize interrupts
5. Enable hardware interrupts
6. Initialize timer
7. Initialize C++ support
8. Register status bar timer callback
9. Start virtual machine (and run OS)
10. Disable hardware interrupts
11. Print stop message

Memory Allocation

The kernel features a simple malloc structure that allocates memor between 0x400000 and 0x800000 (4 MB heap space).

The allocator can be configured to "secure" its allocation list with a magic number by defining USE_MAGIC_SECURED_MALLOC in config.h.

It also features a surveillance mode that logs all allocations / frees. It can be enabled by defining USE_VERBOSE_MALLOC and USE_VERBOSE_FREE in config.h.

Interrupt Mapping

Number	Interrupt
0x00 - 0x1F	System Interrupts
0x20 - 0x27	IRQ0 - IRQ7
0x28 - 0x2F	IRQ8 - IRQ15

Virtual Machine Architecture

Type Format

The virtual machine supports 5 base types:

Name	Description	Size
Void	A type that specifies a non-existent value.	0
Int	An integral number.	32
Real	A decimal number.	32/64¹
Text	A string value containing a sequence of bytes.	any²
Bool	A boolen value, either true or false.	1³

¹) A real number is either a float (32 bits) or a double (64 bits) depending on the configuration in types.hpp.

²) A string value is sized the length of the string in bytes plus the size of the length specifier (32 bits).

³) A boolean is representet as a 1 bit value but due to architecture limitations it is stored with 32 bits.

Variable Format

Variables are stored in a type+pointer format where type stores the type of the variable and pointer either a pointer to the variable value or, if type is a pointer type, the pointer value of the variable.

Native Interoperation

The virtual machine features a NativeMethod that allows calling a C function (result_t __cdecl name(args...)) from the virtual machine. Parameters are passed as the VM types declared in types.hpp, except for ker::String which is passed as a const char*.

Native Modules

The kernel provides native modules that can be used to interoperate with the kernel and the system hardware.

Following modules are planned:

Timer

The timer module provides interaction with the system timer.

Memory

The memory module allows allocating and freeing memory as well as getting predefined named memory areas.

Interrupts

The interrupt module allows handling interrupts by the current module.

OS Architecture

To be done.

trainScript

trainScript is the language of trainOS. All programs are written in trainScript and are compiled on execution.

trainScript programs are executed in the kernels virtual machine and can be restricted in their feature set.

Example

This example first prints "Hello World!", then counts to 60 and displays every pair of (n, 60 - n).

OBJ timer   : "/sys/timer";
OBJ console : "/sys/console";

PUB main() | i : INT, str : TEXT
BEGIN
	0 → i;
	"Hello " → str;
	console.printStr(str + "World!");
	WHILE ((i + 1) → i) <= fun() DO
	BEGIN
		console.print2Int(i, fun() - i);
		timer.sleep(2);
	END
END

PUB fun() → i : INT
BEGIN
	60 → i;
END

Features

trainScript is a pure imperative language with strict typing. Operators can only be called on variables of the same type, there is no auto-casting (which means you need to write 0.0 instead of 0 for getting a REAL constant).

It also features the concept of "Modules" and "Objects". Each trainScript file represents a module that exports a set of methods. An object is an imported module instance. So it is possible to use a module multiple times in another module.

Types

trainScript features 4 base types: INT, REAL, BOOL and TEXT. The description of the four types can be found in the section Virtual Machine Architecture: Type Format.