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mateusz.vi |
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; WORK IN PROGRESS TEMPLATE! NOT USED YET
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; int 24h handler, part of SvarCOM. MIT license.
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; Copyright (C) 2022 Mateusz Viste
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;
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; this is an executable image that can be set up as the critical error handler
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; interrupt (int 24h). It displays the usual "Abort, retry, fail" prompt.
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;
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org 0 ; this code does not have a PSP, it is loaded as-is into a memory
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; segment
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; === CRIT HANDLER DETAILS ====================================================
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;
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; *** ON ENTRY ***
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;
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; upon entry to the INT 24h handler, the registers are as follows:
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; BP:SI = addr of a "device header" that identifies the failing device
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; DI = error code in lower 8 bits (only for non-disk errors)
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; AL = drive number, but only if AH bit 7 is reset
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; AH = error flags
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; 0x80 = reset if device is a disk, set otherwise
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; all the following are valid ONLY for disks (0x80 reset):
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; 0x20 = set if "ignore" action allowed
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; 0x10 = set if "retry" action allowed
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; 0x08 = set if "fail" action allowed
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; 0x06 = disk area, 0=sys files, 1=fat, 10=directory, 11=data
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; 0x01 = set if failure is a write, reset if read
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;
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; within the int 24h handler, only these DOS functions are allowed:
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; 01H-0CH (DOS character I/O)
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; 33H (all subfns are OK, including 3306H get DOS version)
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; 50H (set PSP address)
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; 51H and 62H (query PSP address)
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; 59H (get extended error information)
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;
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;
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; *** ON EXIT ***
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;
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; After handling the error, AL should be set with an action code and get back
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; to DOS. Available actions are defined in AH at entry (see above). Possible
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; values on exit are:
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; AL=0 ignore error (pretend nothing happenned)
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; AL=1 retry operation
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; AL=2 abort (terminates the failed program via int 23h, like ctrl+break)
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; AL=3 return to application indicating a failure of the DOS function
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;
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; A very basic "always fail" handler would be as simple as this:
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; mov al, 3
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; iret
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; =============================================================================
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; save registers so I can restore them later
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push ah
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push bx
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push cx
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push dx
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pushf
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; disk errors product this message:
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; CRITICAL ERROR - DRIVE A: - READ|WRITE FAILURE
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; (A)bort, (R)etry, (F)ail
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;
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; non-disk errors produce this:
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; CRITICAL ERROR #errno
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; restore registers and quit the handler
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popf
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pop dx
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pop cx
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pop bx
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pop ah
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iret
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; write DX string to stderr
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write_dx_str_to_stderr:
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push ax
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push bx
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push cx
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mov ah, 0x40 ; write to file -- am I sure I can use this function safely?
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mov bx, 2 ; stderr
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mov cx, 1 ; one byte
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int 0x21
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pop cx
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pop bx
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pop ax
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