system processor is in real mode. Because this is the case at system boot time and un-der normal DOS operation, simple segment arithmetic is normally used to convertES:BX to a physical address usable by the AHA-1740A/1742A:Physical address = (Segment * 16) + OffsetThe 386 processor, however has given rise to a large number of software productswhich use the 386 memory management feature to provide virtual 8086 operation.The address passed in ES:BX still appears valid to the BIOS, but the processor is notin fact in real mode, and normal segment arithmetic will not yield the correct physi-cal location of the data buffer. This issue has been addressed by IBM ® andMicrosoft ® in a specification known as Virtual DMA Services (VDS). This specifica-tion provides a method to the ROM BIOS for determining the physical location of abuffer given the selector:offset as well as its layout (it may not be contiguous). TheAHA-1740A/1742A BIOS will fully support the VDS specification so that 386 memorymanagement programs will be able to run without a driver as long as they are VDScompliant. In ISA Standard Mode, however, VDS will not be supported and a driverwill be necessary to support VDS.BIOS Command Return CodesAny Int 13h request, upon completion, returns control to the requestor with a returncode set in the carry flag and a status code placed in AH. The carry flag is set asfollows:CF=1 Error or unusual condition. Status is nonzero.CF=0 Command completed normally. Status is zero.Status code definitions returned in register AH, as well as the mapping from SCSI er-ror conditions, are provided in the following table:AH (Hex) Definition00 No Error: Operation completed successfully01 Invalid Function Request: The Int 13h function code provided was notvalid or the drive number was out of range.02 Unable to Read Address Mark: One of the following additional sensecodes (SCSI ASC) was presented in the sense information returned by thetarget:12h - No AM Found on Data Field21h - Illegal Logical Block Address03 Write Protect Error: Returned SCSI ASC:27h - Data Protect04 Read Error: Returned SCSI ASC:14h - No Record Found16h - Data Sync Error10 Uncorrectable ECC Error: Returned SCSI ASC:10h - ID ECC Error11h - Unrecovered Read Erroradaptec AHA-1740A/1742A/17447-6
