mirror of https://github.com/marqs85/ossc.git
134 lines
4.8 KiB
C
134 lines
4.8 KiB
C
//
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// Copyright (C) 2020 Ari Sundholm <megari@iki.fi>
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//
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// This file has been contributed to the Open Source Scan Converter project
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// developed by Markus Hiienkari Markus Hiienkari <mhiienka@niksula.hut.fi>
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// and other members of the retro gaming community.
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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//
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#include <string.h>
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#include "fat16_export.h"
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/*
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* The beginning of the boot sector, along with the BPB.
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*/
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static const alt_u8 bootsec_beg_bpb_16[27] = {
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0xeb, 0x00, 0x90, /* 0x000: Code (x86 short jump + NOP) */
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0x4d, 0x53, 0x57, 0x49, 0x4e, 0x34, 0x2e, 0x31, /* 0x003: OS Name */
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0x00, 0x02, 0x04, 0x80, 0x00, 0x02, 0x00, 0x08, /* 0x00B: Bios Parameter Block */
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0x00, 0x80, 0xf8, 0x20, 0x00, 0x3f, 0x00, 0xff,
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};
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/*
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* The rest of the boot sector before the boot code and terminator.
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* Offsets 0x024 to 0x03d, inclusive.
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*/
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static const alt_u8 bootsec_after_bpb_16[26] = {
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/* Zeros */ 0x80, 0x00, 0x29, 0xf4, /* 0x024...0x027 */
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0xcf, 0xc6, 0x04, 0x4f, 0x53, 0x53, 0x43, 0x50, /* 0x028...0x02f */
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0x52, 0x4f, 0x46, 0x49, 0x4c, 0x53, 0x46, 0x41, /* 0x030...0x037 */
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0x54, 0x31, 0x36, 0x20, 0x20, 0x20, /* Zeros */ /* 0x038...0x03d */
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};
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/*
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* After this, we have the boot code (448 bytes) and sector terminator
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* (2 bytes). The former will be left zeroed-out and the latter will
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* be generated.
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*/
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/* Generates a FAT16 boot sector.
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* buf must be at least FAT16_SECTOR_SIZE bytes long,
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* and is assumed to be pre-zeroed.
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*/
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void generate_boot_sector_16(alt_u8 *const buf) {
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/* Initial FAT16 boot sector contents + the BPB. */
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memcpy(buf, bootsec_beg_bpb_16, sizeof(bootsec_beg_bpb_16));
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/*
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* Then the rest of the boot sector.
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*
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* The boot code is supposed to be 448 bytes filled with 0xf4,
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* but leave it zeroed out to keep the code smaller. This may
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* be a deviation from the FAT16 spec, but should be harmless
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* for our purposes.
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*/
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memcpy(buf + 36, bootsec_after_bpb_16, sizeof(bootsec_after_bpb_16));
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/* RISC-V is little-endian, so do a 16-bit write instead. */
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*((alt_u16*)(buf + 510)) = 0xaa55U;
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}
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/* The fixed 'preamble' of a FAT on a FAT16 volume. */
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static const alt_u32 fat16_preamble = 0xfffffff8U;
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/*
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* Generate a FAT.
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* The buffer is assumed to be zeroed out and have a size of at least
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* FAT16_SECTOR_SIZE bytes.
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* The number of clusters already written is given as an argument.
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* The function returns the total number of clusters written so far.
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*
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* The intention is to be able to generate and write the FAT in chunks
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* that do not exhaust all the remaining RAM.
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*/
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alt_u16 generate_fat16(void *const buf, const alt_u16 written) {
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alt_u16 cur_ofs = 0;
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const alt_u16 start_cluster = 3U + written;
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alt_u16 *const fat = buf;
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/*
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* The total number of FAT entries to write consists of:
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* 1. The FAT "preamble" (2 entries),
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* 2. The cluster chain of the file (512 entries).
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*
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* The latter needs to contain the chain terminator.
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*/
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const alt_u16 clusters_remaining = PROF_16_CLUSTER_COUNT - written;
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const alt_u16 preamble_compensation = written ? 0 : 2U;
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const alt_u16 clusters_to_write =
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((clusters_remaining > FAT16_ENTRIES_PER_SECTOR)
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? FAT16_ENTRIES_PER_SECTOR
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: clusters_remaining) - preamble_compensation;
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const alt_u16 end_cluster = start_cluster + clusters_to_write;
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const alt_u16 last_fat_cluster = PROF_16_CLUSTER_COUNT + 2U;
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if (!written) {
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*((alt_u32*)fat) = fat16_preamble;
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cur_ofs += sizeof(fat16_preamble)/sizeof(alt_u16);
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}
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for (alt_u16 cluster = start_cluster; cluster < end_cluster; ++cluster) {
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alt_u16 *const cur_entry = fat + cur_ofs;
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/* FAT16 entries are 16-bit little-endian. */
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if (cluster == last_fat_cluster) {
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/* At the last cluster, write the chain terminator. */
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*cur_entry = 0xffffU;
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}
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else {
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*cur_entry = cluster;
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}
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++cur_ofs;
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}
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return end_cluster - 3U;
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}
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const alt_u8 prof_dirent_16[PROF_DIRENT_16_SIZE] = {
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0x4f, 0x53, 0x53, 0x43, 0x50, 0x52, 0x4f, 0x46, 0x42, 0x49, 0x4e, 0x20,
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0x00, 0x8e, 0x04, 0xb5, 0x6f, 0x51, 0x6f, 0x51, 0x00, 0x00, 0x17, 0x89,
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0x6f, 0x51, 0x02, 0x00, 0x00, 0x02, 0x10, 0x00,
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};
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