Procedures and Interrupts

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Procedures and Interrupts

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Title: Procedures and Interrupts

1
Procedures and Interrupts

Chapter 5
Stack
Procedure
Software Interrupt
BIOS-level access
DOS-level access
Video Display
Direct Video access

2
The Stack

The stack resides in the stack segment (in main
memory) whos segment number is in the SS
register
The SP register holds the offset address of the
last element added to the stack
If the stack was allocated with the directive
.STACK 100h
Then SP should, initially, contain 100h (pointing
to the top of the empty stack)

3
PUSH (16-bit case)

PUSH source
will decrement SP by 2
and copy the content of source into word at SSSP
little endian low order byte at lowest offset
Ex (see figure)
mov ax,06
push ax
mov ax,0A5h
push ax
This is for a source of type word (reg16 or
mem16).
imm16 are allowed only on 286 and later
processors)

4
PUSH (32-bit case)

With a 32-bit operand (.386 directive)
push source
Decrements SP by 4 and copies the content of
source into the double word at address SSSP
Little endian convention. Ex
mov eax,12345678h
push eax
will decrease SP by 4 and will move
78h at SSSP
56h at SSSP1
34h at SSSP2
12h at SSSP4

5
POP

The POP instruction undoes the action of PUSH
POP destination
For a 16-bit destination operand
the word at SSSP is copied into destination
SP is incremented by 2
For a 32-bit destination operand
the dword at SSSP is copied into destination
SP is incremented by 4
The destination operand cannot be imm

6
Ex saving and restoring registers

.data
message db Hello world
.code
push ax save AX
push dx save DX, SP points to copy of DX
mov ah,9
mov dx, offset message
int 21h prints message
pop dx restore DX
pop ax restore AX

7
More Saving and Restoring

PUSHA (.286) pushes AX, CX, DX, BX, SP, BP, SI,
DI on stack and POPA pops the same registers in
reverse order
PUSHAD (.386) pushes EAX, ECX, EDX, EBX, ESP,
EBP, ESI, EDI on stack and POPAD pops the same
registers in reverse order
PUSHF and POPF pushes and pops the FLAGS register
onto and from the stack
PUSHFD and POPFD (.386) pushes and pops the
EFLAGS register onto and from the stack

8
Procedures

Procedures are defined like this
name PROC type
... set of instructions...
RET
name ENDP
The type is either NEAR or FAR
To transfer control to the procedure name we
do
CALL type PTR name
RET transfers control to the instr. following
CALL
The default for type and type PTR is
NEAR for memory models tiny, small, compact
FAR for memory models medium, large, huge

9
CALL RET (NEAR Procedures)

Upon a CALL to a NEAR procedure
SP is decremented by 2
The content of IP is copied at SSSP
this is the offset address of the instruction
following CALL (where the procedure must return)
The offset address of the first instruction in
the called procedure is copied into IP
this will thus be the next instruction to execute
Upon a RET from a NEAR procedure
the word at SSSP is popped into IP (so that SP
is automatically incremented by 2)
(the instruction pointed by IP is then executed)

10
CALL RET (NEAR Procedures)
11
CALL RET (FAR Procedures)

Upon a CALL to a FAR procedure
CS and then IP are pushed onto the stack
this is the segmentoffset address of the
instruction following CALL (where the procedure
must return)
The segmentoffset address of the first
instruction in the called procedure is copied
into CSIP
this will thus be the next instruction to execute
A RET from a FAR procedure effectively does
POP IP
POP CS
Hence the instruction at CSIP is then executed

12
CALL RET (FAR Procedures)
13
When does a procedure needs to be FAR?

A NEAR CALL is faster than a FAR CALL
Procedures located in the same segment as the
code that CALLs them can be of type NEAR
since the code segment number (in CS) is the same
for both the procedure and the caller
Procedures located in a different segment than
the code that CALLs them must be of type FAR
since the procedure and the caller have a
different code segment number

14
Using Procedures in irvine.lib

Separately assembled procedures under the .model
small will be combined, by the linker, into the
same code segment
this is the case for the procedures in irvine.lib
so use a NEAR call to call these procedures
you should also use .model small for your code
that call procedures in irvine.lib
other memory models will be used when linking
with high level language (HLL) procedures (chap 9
and 13)

15
Passing Arguments to Procedures

Arguments can be passed to procedures via
the stack this is the technique used in HLLs. We
will use this only later (chap 9)
global variables the scope of a variable is the
.ASM file into which it is defined
must use PUBLIC and EXTRN directive to make them
visible to other .ASM files
contrary to modular programming practice
registers fastest way to pass arguments

16
Using Procedures

When a procedure returns to the caller it should
preserve the content of the registers (except
those used to return a value)
should save first the content of the registers
that it will modify and restore them just before
returning to the caller
Caution on stack usage
SP points to the return address when entering the
procedure. Make sure that this is the case just
before executing RET !!
ProcEx.html

Break ...
17
Interrupts

The term interrupt is used in many different ways
A hardware interrupt is a signal generated by any
part of the hardware that needs immediate
attention of the processor
A software interrupt (sometimes called a Trap) is
a call to an Interrupt Service Routine (ISR) of
the Operating System (here either DOS or BIOS)
produced by the instruction INT n in a program
A processor exception is an automatically
generated trap in response to an exceptional
condition (abnormal program execution). Ex
divide overflow, coprocessor not available...

18
Hardware Interrupts

When a hardware component (ex a peripheral
device) needs CPU attention, the controller
associated with this component sends a Interrupt
Request (INTR) signal to the CPU and puts an
Interrupt Number (0 to FFh) onto the data bus
The CPU uses this interrupt number to index the
interrupt vector table (IVT) located at physical
addresses 00000h to 003FFh (pp.33)
Each entry of this table, called an interrupt
vector, contains the segmentoffset address of
the Interrupt Handler (ISR) servicing that
interrupt.
To service an interrupt, the CPU transfers
control to the corresponding ISR

19
The Interrupt Vector Table (IVT)

Each entry of the IVT occupies 4 bytes
At entry 0 of the IVT we have the offset address
and then the segment address of the ISR handling
INT 0
At entry n of the IVT we have the offset address
and then the segment address of the ISR handling
INT n

20
Interrupt Processing

The same mechanisms are used to handle all types
of interrupts (hardware, software, exception)
When an interrupt occurs
The CPU pushes the FLAGS register onto the stack
The CPU pushes onto the stack the far
(segmentoffset) return address (ie that of the
next instruction)
From the interrupt number N, the CPU fetches the
Nth entry of the IVT and transfers control to
that ISR
The ISR execute a IRET instruction to return
control to the program at the point of
interruption (this pops off the stack the far
return address and the FLAGS register)

21
Ex using INT 10h BIOS video services
22
Interrupt Service Routines

A ISR is like a procedure except that
a transfer to a ISR pushes FLAGS in addition to
a far return address
a ISR returns with IRET instead of RET
But since the point of interruption can occur
anywhere in a program, it is crucial for a ISR to
not modify the content of any register
How to write a ISR and how to initialize the
corresponding entry in the IVT? (chap 15)
For now let us examine what are the ISRs that are
provided by DOS and BIOS (and how to use them) to
perform I/O operations

23
Common Software Interrupts

Int 10h Video Services
Int 16h Keyboard Services
Int 17h Printer Services
Int 1Ah Time of Day
Int 1Ch User Timer Interrupt
Int 21h DOS Services

24
MS-DOS Function Calls

A MS-DOS function is called upon the execution
of INT 21h
The actual function to be performed depends on
the function number stored in AH
about 90 different functions are supported
We have already seen functions 01h, 02h, 09h and
4Ch
We now briefly view some other functions
see more details in section 5.5 of your textbook

25
Output Functions

02h Character Output
05h Printer Output
06h Direct Output
09h String Output

26
Input Functions

01h Filtered Input With Echo
06h Direct Input Without Waiting
07h Direct Input, No Ctrl-Break
08h Direct Input with Ctrl-Break
0Ah Buffered Input
0Bh Get Input Status
0Ch Clear Input Buffer, Invoke Input Function
3Fh Read From File or Device

27
Single Character input (DOS)

For all these functions, the next character in
the keyboard buffer is stored in AL
Wait for keystroke function 6 (with DLFFh)
always returns even when the buffer is empty
Function 1 and 8 will return control to DOS when
Ctrl-Break is entered

28
Ex AH06h clear_keyboard

Clear_keyboard proc
push ax
push dx
L1
mov ah, 6
mov dl, 0FFh
int 21h
jnz L1
pop dx
pop ax
ret
clear_keyboard endp

29
Buffered Input (DOS)

Function 0Ah reads (from stdin) a string of up to
255 characters and stores it in a buffer
User input is terminated with 0Dh (CR)
Non ASCII keys (ex PgUp, arrows, Fn...) are
filtered out and Ctrl-Break is active
DX contains the offset of the Buffer
1st char max number of char allowed (including
0Dh)
2nd char number of chars actually entered
(excluding 0Dh)

30
Ex Using buffered input function 0Ah

.data
keyboardArea label byte
maxkeys db 32 max chars allowed
charsInput db ? of chars actually
entered
buffer db 32 dup('0') holds input
string
.code
mov ah,0Ah
mov dx,offset keyboardArea
int 21h
the CR (0Dh) is the last char entered in the
buffer

31
Date/Time Functions
cx year dh month dl day

2Ah Get Date
2Bh Set Date
2Ch Get Time
2Dh Set Time

ch hour cl minute dh second
Break ...
32
Keyboard Keys

ASCII keys
those that have an ASCII code letters, digits,
punctuations, arithmitics, Esc, CR, Bksp, Tab
Shift Keys
normally used in combination with another key
left and right shifts, Caps Lock, Ctrl, Alt, Num
Lock, Scroll Lock
Function Keys
used in programs to perform special functions
F1-F12, arrows, Home, PgUp, PgDn, End, Ins, Del

33
Scan Codes

Only ASCII keys have an ASCII code but all keys
have a SCAN CODE (1byte). See scancodes.html
When we strike a key
The keyboard interrupts (INT 9h) the CPU and
sends the Scan Code to I/O port 60h
The BIOS INT 9h reads this I/O port and uses the
scan code to index a table to get the ASCII code.
Both codes are sent to the keyboard buffer only
if it is not a shift key (used alone)
For each word in the keyboard buffer
low byte ASCII code of the key, or 0 if it is
not an ASCII key
high byte Scan Code of key

34
BIOS input function INT 16h

When AH10h, INT 16h will load AX with the next
word in the keyboard buffer
mov ah,10h
int 16h AH Scan Code, AL ASCII code
The input character will not be echoed on screen
Useful for reading (and identify) the function
key pressed by the user
they can be identified only with their scan code
Keyboard input cannot be redirected on the DOS
command line (unlike INT 21h)

35
Video Adapters

Screen display is controlled by a video adapter
which consists of
A memory (video buffer) which contains all the
information displayed on screen
A video controller that displays on screen the
content of the video buffer
Typical resolutions (in pixels X pixels)
640 X 480 (standard VGA)
800 X 600 (super VGA)
1024 X 768 (extended VGA)
....(higher resolutions)....

36
Video Modes

We have two classes of video modes
graphic modes used to display arbitrary
graphics, including text (not discussed here)
text modes only characters (from the IBM
extended ASCII character set) can be displayed.
(the subject till the end of chapter)
From the many available text modes (mode 0, 1, 2,
3, 7) we discuss only mode 3 (most important one)
displays text on 80 columns and 25 rows
first row row 0 top of the screen
first column column 0 left of screen
16 colors are available

37
Video Pages

Each character displayed is represented by 1 word
low order byte ASCII code (IBM extended)
high order byte Attribute Byte (specify how the
character will be displayed)
Each of these words is stored in the video buffer
starting at physical address B80000h
One screen of text (80 X 25 X 2 4000 bytes)
requires 1 video page of 4KB
VGA (and better) adapters can hold 8 video pages
page 0 to 7

38
Video Pages (cont.)

only the active page is displayed
the first word of the page displays the character
at the upper left corner (row,column) (0,0)
the second word displays the character at (row,
column) (1,0)
the 3rd word displays the char at (2,0)...
...the last word displays the char at (24,79)
(other pages can be modified while the active
page is being displayed)

39
The Attribute Byte

The foreground bits determine the color of the
character
The background bits determine the color of the
background
The msb of foreground is an intensity bit
The blinking bit applies only to foreground

40
Foreground Colors

Background colors are the same as foreground
colors with msb 0

41
Ways to write on the screen

We can write directly to the video buffer to
display text. See Direct2Videomem.html
this is the fastest method but also the most
complex. Cannot redirect the output with DOS.
We can use DOS INT 21h functions
very slow to go through DOS
Output can be redirected (DOS command line)
We can use BIOS-LEVEL INT 10h functions
faster than DOS but slower than direct access
Cannot redirect the output

42
Some BIOS INT 10h functions