gistfile1.c

/* bfs512: B File System 512

  A simple file system for m35fd compatible discs. 
	- Track free sectors
	- Link sectors to form larger files.
*/

#ifndef DCPUB_LIB_BFS512
#define DCPUB_LIB_BFS512

#include m35fd.dc

/* The structure of a bfs512 disc.

A) Header [6 words]
B) Free Mask [1440 bits or 90 words]
C) File Allocation Table [ 1440 words]
D) File data

Total filesystem overhead : 1536 words or 3 sectors.

Filenames and directories are beyond the scope of this filesystem. A suggested implementation strategy is to
make directories files. Place the root directory at sector 3, the first non-header sector.

*/

#define BFS512_SECTOR_WORDS (M35FD_SECTOR_COUNT / 16)

// File system header. Allocate one of these and keep it for interacting with the filesystem.
struct bfs512_SYSTEM_HEADER
{
	version;
	sector_size;
	num_sectors;
	reserved[3];
	free_mask[BFS512_SECTOR_WORDS];
	allocation_table[M35FD_SECTOR_COUNT];
}

//Load a disc header from an m35fd device.
function bfs512_load_disc(device, header /* Pointer to sizeof(bfs512_SYSTEM_HEADER) words of memory.*/)
{
	m35fd_blocking_read(device, 0, header);
	m35fd_blocking_read(device, 1, header + M35FD_SECTOR_SIZE);
	m35fd_blocking_read(device, 2, header + (M35FD_SECTOR_COUNT * 2));
}

//Find free sector
function bfs512_find_free_sector(header:bfs512_SYSTEM_HEADER)
{
	local word = 0;
	while (word < BFS512_SECTOR_WORDS)
	{
		if (header.free_mask[word] != 0)
		{
			//Which bit is free?
			local bit = 0;
			while (bit < 8)
			{
				local mask = 1 << bit;
				if ((header.free_mask[word] & mask) > 1) return (word * 16) + bit;
				bit += 1;
			}
			//Unreachable
		}
		word += 1;
	}
	return 0xFFFF; //No free space.
}

//Allocate sector
function bfs512_allocate_sector(header:bfs512_SYSTEM_HEADER, sector)
{
	local word = sector / 16;
	local bit = sector % 16;
	local mask = 1 << bit;
	header.free_mask[word] |= mask;
}

//Free sector
function bfs512_free_sector(header:bfs512_SYSTEM_HEADER, sector)
{
	local word = sector / 16;
	local bit = sector % 16;
	local mask = !(1 << bit);
	header.free_mask[word] &= mask;
}

//Link sectors - Also works as unlink (link to 0xFFFF)
function bfs512_link_sectors(header:bfs512_SYSTEM_HEADER, sector, to)
{
	header.allocation_table[sector] = to;
}

//Save header back to the device. If files are modified and the disc is removed before this header is saved,
//files on the disc may be corrupted.
function bfs512_save_header(device, header)
{
	m35fd_blocking_write(device, 0, header);
	m35fd_blocking_write(device, 1, header + M35FD_SECTOR_SIZE);
	m35fd_blocking_write(device, 2, header + (M35FD_SECTOR_SIZE * 2));
}

//Format a header block. To format a disc, allocate a header, call this function, and then save the header. 
function bfs512_format_header(header)
{
	local i = 0;
	while (i < 6)
	{
		header[i] = 0x0000;
		i += 1;
	}

	//Prime free_mask
	while (i < (6 + BFS512_SECTOR_WORDS))
	{
		header[i] = 0xFFFF;
		i += 1;
	}

	//Prime sector links
	while (i < (M35FD_SECTOR_SIZE * 3))
	{
		header[i] = 0xFFFF;
		i += 1;
	}
}

#endif