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 bytes:[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 args:[u16]*`: runs the program in the current segment with arguments.
* `i ax:u16 args:[u16]*`: call an int 0x80 function with given AX and stack args. 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_


## Examples

### Launching mushroom client

```
s 200
l mushroom.com
r
```

### FTP a file and store it to the floppy disk

```
s 200
l ftpget.com
```

* switch to segment _0x200_
* load file _ftpget.com_ to _cur-seg:0100_

Here you need to run this on the host:
```
python ftpserve.py src/hello.com
```

Then on polOS:
```
r f000 200
s 0
6000
w 48454c4c4f202020434f4d
i 0800 6000 f000 200
```

* run previously loaded `ftpget.com`: store up to _0x200_ (512) bytes to address _0000:f000_
* set _cur-seg_ to _0x0000_
* set _cur-add_ to _0x6000_
* write file name in ascii: `HELLO   COM` to _0000:6000_
* call int 0x80 0x08 (writefile) with args: _fname_, _addr_, _size_


We can now test that the program got transferred correctly:
```
s 300
l hello.com
r
```

### But where is my `ls` command?

You can just inspect memory at _0000:1c00_, that's the root directory. Each entry is _0x20_ (32) bytes long and starts with 11 characters that constitute the file name.

## 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)