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Parameter Passing
All parameters passed to and from the BIOS routines go through
the 8088 registers. The prologue of each BIOS function indicates
the registers used on the call and the return. For the memory size
example, no parameters are passed. The memory size, in lK-byte
increments, is returned in the AX register.
If a BIOS function has several possible operations, the AH
register is used as input to indicate the desired operation. For
example, to set the time of day, the following code is required:
MOV AU,1
;function is to set time of day.
MOV CX,UIGU_COUNT ;establish the current time.
MOV DX,LOW_COUNT
INT IAU
To read the time of day:
MOV AU,O
INT IAU
;set the time.
;function is to read time of day.
;read the timer.
Generally, the BIOS routines save all registers except for AX and
the flags. Other registers are modified on return only if they are
returning a value to the caller. The exact register usage is in the
prologue of each BIOS function.
Int
Address
Name
BIOS Entry
0
0-3
Divide by Zero
D11
1
4-7
Single Step
D11
2
8-B
Nonmaskable
NMI INT
3
C-F
Breakpoint
D11
-
4
10-13
Overflow
D11
5
14-17
Print Screen
PRINT SCREEN
6
18-1B
Reserved
D11
7
1C-1F
Reserved
Dl"l
8
20-23
Timer
TIMER INT
9
24-27
Keyboard
KB INf
A
28-2B
Reserved
D1T
B
2C-2F
Communications
D11
C
30-33
Communications
D11
8088 Software Interrupt Listing (Part 1 of 2)
5-4
System BIOS
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