Category Archives: BIOS

IBM DOS 5.0 QBASIC hangs on non-IBM systems

Much like its predecessors, IBM DOS 5.0 has no trouble running on clone systems, as long as they’re sufficiently compatible. That includes virtual machines. However, the QBASIC.EXE (and consequently EDIT.COM) program dated May 9, 1991 consistently hangs on any non-IBM … Continue reading

Posted in BIOS, DOS, PC history | 20 Comments

Detecting floppy drive presence and type

Detecting floppy drives is surprisingly tricky and poorly understood. The difficulty lies mainly in the interface between the floppy disk controller (FDC) and the actual drive(s). The interface isn’t particularly complex, but it isn’t straightforward, and it does not follow … Continue reading

Posted in BIOS, PC architecture | 1 Comment

Interrupt 68h and EMM386

While working with a modified BIOS image in a virtual machine, I ran into mysterious hangs when trying to load an old version of EMM386.SYS from MS-DOS 4.01. A newer version of EMM386 did not exhibit the problem. A quick … Continue reading

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The Fixed Disk Parameter Table

The Fixed Disk Parameter Table, or FDPT, is a structure primarily used by the BIOS in IBM compatible computers, but is also of critical importance to some (especially older) operating systems which do not use the BIOS. The FDPT was … Continue reading

Posted in BIOS, PC architecture, Virtualization | 3 Comments

Geometry Problems

When introducing hard disk support in the PC/XT back in early 1983,┬áIBM made a very unfortunate design decision: the information about drive geometry was exposed in the BIOS, and even worse, in the boot sector stored on the disk. SCSI … Continue reading

Posted in BIOS, PC architecture, Virtualization | 2 Comments