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Added more info about DOS 2.0 BPBs

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Jeff Parsons 2017-06-01 16:42:46 -07:00 • committed by Jeff Parsons
commit 20f8bdf141
2 changed files with 21 additions and 12 deletions
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15
modules/diskdump/lib/diskdump.js
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@ -541,14 +541,15 @@ DiskDump.aDefaultBPBs = [
0x08, 0x00, // 0x16: sectors per FAT (8)
//
// Wikipedia (http://en.wikipedia.org/wiki/File_Allocation_Table#BIOS_Parameter_Block) implies everything past
// this point was introduced post-DOS 2.0. I think that's wrong, because I just formatted a diskette with PC-DOS 2.0
// and it properly initialized the next 3 fields as well. TODO: Investigate PC-DOS 2.0 BPB behavior.
// this point was introduced post-DOS 2.0. However, DOS 2.0 merely said they were optional, and in fact, DOS 2.0
// FORMAT always initializes the next 3 words. A 4th word, LARGE_SECS, was added in DOS 3.20 at offset 0x1E,
// and then in DOS 3.31, both HIDDEN_SECS and LARGE_SECS were widened from words to dwords.
//
0x11, 0x00, // 0x18: sectors per track (17)
0x04, 0x00, // 0x1A: number of heads (4)
//
// PC-DOS 2.0 actually stored 0x01, 0x00, 0x80, 0x00 here, so you can't always assume that anything past offset
// 0x1E is part of the BPB. TODO: Investigate PC-DOS 2.0 BPB behavior.
// PC-DOS 2.0 actually stored 0x01, 0x00, 0x80, 0x00 here, so you can't rely on more than the first word.
// TODO: Investigate PC-DOS 2.0 BPB behavior (ie, what did the 0x80 mean)?
//
0x01, 0x00, 0x00, 0x00 // 0x1C: number of hidden sectors (always 0 for non-partitioned media)
]
@ -2652,10 +2653,10 @@ DiskDump.prototype.convertToJSON = function()
if (iBPB >= 0) {
if (fBPBExists) {
/*
* In deference to the PC-DOS 2.0 anomaly noted above (see "Investigate PC-DOS 2.0 BPB behavior" above),
* we limit the following byte verification to the offset of LARGE_SECS, less two bytes.
* In deference to the PC-DOS 2.0 BPB behavior discussed above, we stop our BPB verification
* after the first word of HIDDEN_SECS.
*/
for (i = DiskAPI.BPB.SECTOR_BYTES; i < DiskAPI.BPB.LARGE_SECS - 2; i++) {
for (i = DiskAPI.BPB.SECTOR_BYTES; i < DiskAPI.BPB.HIDDEN_SECS + 2; i++) {
var bDefault = DiskDump.aDefaultBPBs[iBPB][i];
var bActual = this.bufDisk.readUInt8(offBootSector + i);
if (bDefault != bActual) {