1. CS-401 Computer Architecture & Assembly Language Programming
Lecture-36
Secondary Storage

2. Lets revise the last lecture

3. Logical view of secondary storage

4. int 0x21 DOS Services Create Truncate File AH = 0x3C
CX = file attribute
DS:DX = ASCII file name
Return
On success
CF = clear
AX = file handle
On error
CF = set
AX = error code

5. int 0x21 DOS Services Open Existing File AH = 0x3D
AL = access and sharing mode
DS:DX = ASCII file name
CL = attribute mask
Return
On success
CF = clear
AX = file handle
On error
CF = set
AX = error code

6. int 0x21 DOS Services Close File AH = 0x3D BC = file handle Return
On success
CF = clear
AX = undefined
On error
CF = set
AX = error code

7. int 0x21 DOS Services Read File AH = 0x3F BX = file handle
CX = number of bytes to read
DS:DX = buffer
Return
On success
CF = clear
AX = number of bytes actually read
On error
CF = set
AX = error code

8. int 0x21 DOS Services Write File AH = 0x40 BX = file handle
CX = number of bytes to write
DS:DX = buffer
Return
On success
CF = clear
AX = number of bytes actually written
On error
CF = set
AX = error code

9. int 0x21 DOS Services Delete File AH = 0x41 DS:DX = ASCII file name
CL = attribute mask
Return
On success
CF = clear
AX = undefined
On error
CF = set
AX = error code

10. Seek –Set Current File Position
int 0x21 DOS Services
Seek –Set Current File Position
AH = 0x42
AL = origin of move (00=start,01=current,02=end)
BX = file handle
DS:DX = offset from origin
Return
On success
CF = clear
DX:AX = new file position
On error
CF = set
AX = error code

11. int 0x21 DOS Services Get File Attributes AH = 0x43 AL = 0x00
DS:DX = ASCII file name
Return
On success
CF = clear
CX = file attributes
On error
CF = set
AX = error code (#01680)

12. int 0x21 DOS Services Set File Attributes AH = 0x43 AL = 0x01
CX = file attributes
DS:DX = ASCII file name
Return
On success
CF = clear
AX = undefined
On error
CF = set
AX = error code

13. PSP

14. PSP 0x00 – 0x7F 0x80 0x80-(0x80+count) Count DOS Information
Chars

15. int 0x21 DOS Services Exec- Load and Execute AH = 0x4B
AL = type (0x00 load and exec, 0x01 load)
DS:DX = ASCII file name
ES:BX = parameter block
Return
On success
CF = clear
On error
AX = error code

16. int 0x21 DOS Services 0x01 0x02 0x06 0x0A 0x0E 0x12 0x16 Command Tail
First
FCB
Second
FCB
SS:SP
SS:IP
Environment

17. int 0x21 DOS Services Allocate Memory AH = 0x48
BX = number of paragraphs
Return
On success
CF = clear
AX = segment of memory block
On error
CF = set
AX = error code
BX = largest available block

18. int 0x21 DOS Services Free Memory AH = 0x49
ES = segment of block to free
Return
On success
CF = clear
On error
CF = set
AX = error code

19. int 0x21 DOS Services Resize Memory Block AH = 0x4A
BX = number of paragraphs
ES = segment of block
Return
On success
CF = clear
On error
CF = set
AX = error code
BX = maximum paragraphs available