User`s guide

Error Code 255 is returned if a physical error is encountered and the BDOS error
mode is Return Error mode, or Return and Display Error mode (see Function 45). If
the error mode is the default mode, a message identifying the physical error is dis-
played at the console, and the calling program is terminated. When a physical error
is returned to the calling program, register H contains one of the following error
codes:
01 Disk I/O error
02 Read-Only disk
03 Read-Only file or
File open from user 0 when
the current user number is non-zero or
File password protected in Write mode
04 Invalid drive error
On all error returns, except for physical error returns, A = 255, Function 21 sets
register H to the number of records successfully written before the error was encoun-
tered. This value can range from 0 to 127 depending on the current BDOS Multi-
Sector Count. It is always set to zero when the Multi-Sector Count is set to one.
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BDOS Function Calls CP/M 3 Programmers Guide