5150 stuff
==========

## Layout of a bootable polOS floppy disk

The floppy disk is FAT12 formatted. It must contain at least the following items:

* _fat12boot.bin_: the boot sector. It loads _polio.com_ from a FAT12-formatted
    drive 0 to address _0x0600_ and jumps to it.
* _polio.com_: the basic OS functions. It serves all interrupt 0x80 functions
    (see below) and loads _polmon.com_ to address _0x1200_, then jumps to it.
* _polmon.com_: a barebones memory utility. See below for all it can do.

## polOS int 0x80

* function in AH
* optional argument in AL
* other arguments on the stack
* return value in AX

| AH | AL | stack arguments | return | description |
|-------------|--------------|-----------------|-------------|
| _0x01_ | | _addr_, _c_, _h_, _s_ | error code in AH | __readsector__: reads 1 floppy sector at _c/h/s_ into _0000:addr_ |
| _0x02_ | | _addr_, _c_, _h_, _s_ | error code in AH | __writesector__: writes 1 floppy sector at _c/h/s_ from _0000:addr_ |
| 0x03 | | _c_, _h_ | error code in AH | __formattrack__: formats 1 track at c/h |
| 0x04 | | _fname_, _addr_ | non-zero if error | __readfile__: reads file with name at _ds:fname_ into _ds:addr_ |
| 0x05 | | _addr_, _size_ | bytes received, < 0 if error | __recvpara__: receive at most _size_ bytes  from parallel port into _ds:addr_ |
| 0x06 | | _addr_, _size_ | bytes sent, < 0 if error | __sendpara__: send at most _size_ bytes to parallel port from _ds:addr_ |
| 0x07 | _until-idle_ | | 0 if idle | __runpara__: run a round of parallel comms if _AL=0_, keep going until idle if _AL!=0_ |
| 0x08 | | _fname_, _addr_, _size_ | error code in AH | __writefile__: writes _size_ bytes from _ds:addr_ into file with name at _ds:fname_ |


## polmon commands

Commands are a single letter followed by an optional space and arguments, with optional spaces between them.
A command can be prefixed with up to 4 hex digits, which will then change the current address.

* (no command): dumps 16 bytes at the current address. If no address is given, also increments the current address by 16.
* _w [u8]*_: writes bytes in hex to the current address.
* _k src:u16 size:u16_: copy _size_ bytes of memory from _cur-seg_:_src_ to _cur-seg:cur-addr_
* _s seg:u16_: changes the current segment
* _j_: calls a function at the current address. ABI calling conventions below.
* _l file:str_: loads a program with given name into the current segment, address _0x0100_.
* _r [u16]*_: runs the program in the current segment with arguments.
* _i ax:u16_: call an int 0x80 function with AX register specified. See section above.

The _r_, _j_ and _i_ commands display the value of the AX register after returning to polmon.

### ABI convention for (j)umping to functions

* AX, BX, CX, DX are caller-saved, all others are callee-saved
* first 3 arguments passed in AX, DX and CX, others are pushed in reverse order (former argument in lower memory)
* return value in AX


### ABI for programs

See example in _hello.com_


## Useful stuff

* [Colab](https://colab.research.google.com/drive/1xGKYQJLKyabcSNYOiPumf9_-bbCbXke1?usp=sharing)
* [Arduino pinout](https://docs.google.com/spreadsheets/d/1jgKhr-0MFtY_bFZL9xYwVsxFPt4u_5ItV8Rj7FQ7Kj4/edit)