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100 lines
4.1 KiB
Plaintext
100 lines
4.1 KiB
Plaintext
DIFFERENCES AND NEW ADDITIONS
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TO THE INT 24H HARD ERROR
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HANDLER(s) FOR MSDOS 2.0
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1.) Additional Constraints:
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Under previous versions it was not explicitly stated
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that an INT 24H handler must preserve the ES register.
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It is now required that INT 24H handlers preserve ES.
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When it is desired to ignore an error, the same
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registers must be preserved as when it is desired to
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retry the operation (SS,SP,DS,BX,CX,DX).
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It was not clearly stated in the past, but it was
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true, that only system calls 1-12 can be made by an INT
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24H handler. Making any other calls will destroy the
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DOS stack and thus its ability to retry or ignore an
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error.
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INT 24H Handlers should always return to the DOS
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on a retry, ignore, or abort. Failure to return to the
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DOS will leave the DOS in an unstable state until a non
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1-12 function call is made.
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2.) Additional features:
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Character device errors are now handled by the INT
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24H mechanism. Previously only Disk I/O errors were
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handled by the INT 24H handler. Additional information
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is now passed to the INT 24H handler in the BP and SI
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registers (which need not be preserved).
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BP:SI is a DWORD pointer to the Device Header of
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the device causing the error. Information can be gotten
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from this header as to whether the device is a block
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or character device, and if the device is a character
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device the name of the device can also be obtained. The
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DEVICE-DRIVERS document for 2.0 contains the definition
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of this header format.
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NOTE: AL (drive number for Disk errors) is indeterminate
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on character device errors. Bit 7 of AH is always
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1 for character device errors, previously bit 7 was
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1 only in the case of a bad memory image of the FAT.
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LIST OF INT 24H ERROR CODES PASSED IN DI
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0 Write Protect violation
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1 Unknown Unit NEW
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2 Drive not ready
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3 Unknown command NEW
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4 CRC error
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5 Bad Drive Request Structure length NEW
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6 Seek error
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7 Unknown media NEW
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8 Sector not found
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9 Printer out of paper NEW
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A Write Fault
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B Read Fault NEW
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C General Failure
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As mentioned above BP:SI points to the device header:
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BP:SI->
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+--------------------------------------+
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| DWORD Pointer to next device |
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| (-1 if last device) |
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+--------------------------------------+
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| WORD Attributes |
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| Bit 15 = 1 if char device 0 if blk |
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| if bit 15 is 1 |
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| Bit 0 = 1 if Current sti device |
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| Bit 1 = 1 if Current sto output |
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| Bit 2 = 1 if Current NUL device |
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| Bit 3 = 1 if Current CLOCK dev |
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| Bit 14 is the IOCTL bit (see below) |
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| Bit 13 is the NON IBM FORMAT bit |
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+--------------------------------------+
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| WORD Pointer to Device strategy |
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| entry point |
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+--------------------------------------+
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| WORD Pointer to Device interrupt |
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| entry point |
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+--------------------------------------+
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| 8-BYTE character device name field |
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| Character devices set a device name |
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| For block devices the first byte is |
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| The number of units |
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+--------------------------------------+
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To tell if the error occured on a block or character
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device you must look at bit 15 in the attribute field (WORD
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at BP:SI+4).
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If the name of the character device is desired look at
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the eight bytes starting at BP:SI+10.
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