INT 0x21 - 0x58 - Get/Set Memory Allocation Strategy

Undocumented, DOS 3.0

Request/Call:

reg	value	descr
AH	0x30
AL	var	00: get strategy code 01: set strategy code
BX	var	strategy code (when AL = 01) 00: first fit (default) 01: best fit 02: last fit

Return:

reg	descr
AX	strategy code if CF clear Error if CF is set (see DOS Error)

defaults to first fit, first block large enough is chosen
in first fit, DOS searches the MCB chain from low addresses to high for the first block large enough to fill the request. For best fit, DOS searches all memory blocks for the closest fit. In last fit, DOS starts at high addresses and works downward and uses the high part of the chosen block
any strategy value greater than 2 defaults to 2, but the last value set, even if incorrect, is the value returned by the get strategy

DOS error Codes

Of the following error codes, only error codes 1-12 are returned in AX upon exit from interrupt 21 or 24; The rest are obtained by issuing the “get extended error” function call; see Int 0x21,0x59

Extended DOS Error Code

00	No Error
01	Invalid function number
02	File not found
03	Path not found
04	Too many open files (no handles left)
05	Access denied
06	Invalid handle
07	Memory control blocks destroyed
08	Insufficient memory
09	Invalid memory block address
0A	Invalid environment
0B	Invalid format
0C	Invalid access mode (open mode is invalid)
0D	Invalid data
0E	Reserved
0F	Invalid drive specified
10	Attempt to remove current directory
11	Not same device
12	No more files
13	Attempt to write on a write-protected diskette
14	Unknown unit
15	Drive not ready
16	Unknown command
17	CRC error
18	Bad request structure length
19	Seek error
1A	Unknown media type
1B	Sector not found
1C	Printer out of paper
1D	Write fault
1E	Read fault
1F	General failure
20	Sharing violation
21	Lock violation
22	Invalid disk change
23	FCB unavailable
24	Sharing buffer overflow
25	Reserved
26	Unable to complete file operation (DOS 4.x)
27-3	1 Reserved
32	Network request not supported
33	Remote computer not listening
34	Duplicate name on network
35	Network name not found
36	Network busy
37	Network device no longer exists
38	NetBIOS command limit exceeded
39	Network adapter error
3A	Incorrect network response
3B	Unexpected network error
3C	Incompatible remote adapter
3D	Print queue full
3E	No space for print file
3F	Print file deleted
40	Network name deleted
41	Access denied
42	Network device type incorrect
43	Network name not found
44	Network name limit exceeded
45	NetBIOS session limit exceeded
46	Temporarily paused
47	Network request not accepted
48	Print or disk redirection is paused
49-4	F Reserved
50	File already exists
51	Reserved
52	Cannot make directory entry
53	Fail on INT 24
54	Too many redirections
55	Duplicate redirection
56	Invalid password
57	Invalid parameter
58	Network device fault
59	Function not supported by network (DOS 4.x)
5A	Required system component not installed (DOS 4.x)

DOS Error Code/Classes

Error	Classes
01	Out of resource, out of space, channel, etc
02	Temporary situation, not an error, ex: file lock
03	Authorization, permission denied
04	Internal, system detected internal error
05	Hardware failure, serious problem related to hardware
06	System failure, ex: invalid configuration
07	Application error, inconsistent request
08	Not found, file/item not found
09	Bad format, file/item in invalid format
0A	Locked, file/item interlocked
0B	Media failure, ECC/CRC error, wrong or bad disk
0C	Already exists, collision with existing item
0D	Unknown, classification doesn't exist or is inappropriate

DOS Error Code/Action Codes

Error	Action Codes (in BL)
01	retry, attempt a few more times and re-prompt
02	delay retry, retry a few more times after a pause
03	re-enter input, prompt user to re-enter input
04	abort with cleanup, orderly abort and shutdown
05	immediate abort, exit immediately without cleanup
06	ignore error
07	user intervention, retry after user fixes the problem

Locus

Error	Locus (in CH)
01	unknown
02	block device
03	network
04	serial device
05	memory

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