EEL 3801

1
EEL 3801

• Part IV
• The Assembler

2
OFFSET Operator

• Returns address of variable used as operand.
• Actually, it represents the offset from the
  beginning of the segment.
• The destination operand must be a 16-bit
  register.
• When a register holds an address (offset) rather
  than the value therein, it is a pointer to that
  variable.

3
SEG Operator

• Returns the segment part of a labels address.
• The segment from which the offset is being
  displaced.
• Not the offset.

4
Example

• Used when the base segment register must be
  initialized to a new segment.
• Assume array is in a segment other than what DS
  points to
• push ds Save contents of DS mov ax,seg array
  set DS to seg mov ds,ax of array mov
  bx,offset array set offset
• . compute whatever
• pop ds restore original DS

5
PTR Operator

• Specifies the size of an operand.
• Can override an operands default size
• BYTE PTR (8-bit)
• WORD PTR (16-bit)
• DWORD PTR (32-bit)
• QWORD PTR (64-bit)
• TBYTE PTR (80-bit)

6
LABEL Directive

• Another way to override a variables default
  type.
• Does not allocate more memory
• Just redefines an existing variable.

7
Transfer of Control Instructions

• CPU executes instructions sequentially, in the
  exact order presented.
• Occasionally, we want a slightly different order
  of execution,
• so must tell assembler so that it knows what is
  the next instruction to load and decode prior to
  execution.

8
Transfer of Control Instructions (cont.)

• Two types
• Conditional transfers
• Unconditional transfers.

9
Unconditional Transfers

• Program branches to new location all the time,
  unconditionally.
• New value loaded into the instruction pointer.

10
Conditional Transfer

• Branching only if certain conditions are true.
• CPU interprets true/false condition based on
  content of CX and Flags registers.
• Some examples of these instructions

11
JMP Instruction

• Tells CPU to begin execution at another location.
• New location to be identified by label
• Label translated by assembler into new address.
• If jump is to label in current segment, labels
  offset loaded into the IP register.
• If jump is to another segment, segment address
  additionally loaded into CS.

12
JMP Instruction (cont.)

• Syntax is
• JMP SHORT/NEAR PTR/FAR PTR destination
• where
• SHORT gt destination within 128 to 127 bytes
• NEAR PTR gt destination in same segment
  (default)
• FAR PTR gt destination in another segment
• destination gt a label or 32 bit segment-offset
  address.

13
JMP Instruction (cont.)

• Can move to just about anywhere,
• to same procedure,
• to another procedure,
• to another segment,
• to RAM or ROM
• or completely out of the current program.
• Loop based on JMP will never end.

14
LOOP Instruction

• Repeats a block of instructions a specific number
  of times.
• Uses CX as a counter, and decrements it every
  iteration.
• The number of cycles must be loaded into CX prior
  to the loop taking place.
• Format is LOOP destination

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LOOP Instruction (cont.)

• Destination must be from 128 to 127 bytes away
  from the current location.
• Destination typically behind loop instruction.
• When it gets to LOOP, assembler decrements CX by
  1.
• If not zero, transfers control to destination.

16
Example 1

• mov cx,1280
• next
• mov ah,2 DOS function display char
• mov dl,A
• int 21h Call DOS function on ah
• loop next

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Example 2

• sum_an_array
• mov ax,0
• mov di,offset intarray
• mov cx,4 array size
• read_int
• add ax,di add integer to accum
• add di,2
• loop read_int
• intarray dw 200h, 100h, 300h, 600h

18
Procedures

• It is generally infeasible to write a program
  consisting of long sequence of instructions.
• True in assembly or high level language.
• More likely, blocks of code that do something
  logically related
• complex mathematical calculation,
• setting up or retrieving a data structure,
• printing something
• reading a file.

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Procedures (cont)

• By grouping and interacting these blocks of code,
  a program is much easier to write, debug and
  document.
• Moreover, the blocks of code may be reusable
  either in the same program, or in other programs.
• These blocks of code are called procedures, or
  subroutines.

20
Procedures (cont)

• Procedures or subroutines represent a branching
  of the program control when a call to a
  subroutine is reached in the program execution.
• The assembler must return to the instruction
  after the one that called the subroutine.

21
Procedures (cont)

• The subroutine can be called through the
  instruction CALL followed by the name of the
  subroutine.
• CALL saves address of next instruction to stack,
  depending on whether near or far call.
• This is different from the JMP instruction
• JMP does not require a return to the original
  address from where it was called.

22
Procedures (cont)

• Subroutines that return a value.
• They are typically equated to a label that takes
  on the value resulting from the function
  execution.
• Procedures do not normally return a value.
• PROC and ENDP are the bookends that mark the
  beginning and end of a procedure.

23
Procedures (cont)

• These directives must include the name of the
  procedure, which precedes the directive itself.
• Procedures must include the RET (return)
  instruction to pop old value of address from
  run-time stack.
• Procedures must not be overlapped.
• See example on page 107 of text book.

24
Near and Far Procedures

• The difference between them is how the assembler
  remembers where to return to.

25
Near Call

• Calling procedure and called procedure are in the
  same segment of the program.
• Assembler generates machine code for a near call.
  
• Before branching to the subroutine, the CALL
  instruction preserves the current content of the
  IP register (the instruction pointer) onto the
  run-time stack.

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Near Call (cont)

• Remember that the IP points to the next
  instruction to be executed.
• It then copies the address of the first
  instruction of the called subroutine into the IP
  register.
• Thus, the CPU follows that sequence of
  instructions that begins at that address.

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Near Call (cont.)

• At the end of the subroutine, the RET instruction
  pops the old value of IP into it again from the
  stack.
• Execution then resumes right from where the call
  was made.

28
Far Call

• This happens when the calling and called
  subroutines are in different segments of the
  code.
• The assembler then does a far call.
• Now the CALL instruction preserves both the
  contents of the IP and the CS register onto the
  stack.
• Uses a RETF instruction instead of RET.

29
Nested Calls

• Calls can be nested.
• The CALL instruction keeps the addresses in the
  run-time stack on a last-in-first-out basis, so
  there is no problem.
• The RET instructions are called in reverse order.
• See example on page 113, Figure 5.7.

30
Interrupts

• There are reasons why the CPU needs to be
  interrupted from its activity.
• Two types of interrupts
• Hardware interrupts
• Software interrupts

31
Hardware Interrupts

• Are signals from other parts of the system
  (hardware) that something needs immediate
  attention from the CPU.
• These signals are generated by the Interrupt
  Controller, a chip called the 8529IC.
• They allow for important events occurring in the
  background to be noticed by the CPU and
  immediately acted upon.

32
Hardware Interrupts - Examples

• Examples of hardware interrupts are inputs that
  would be lost if not read and processed quickly.
  
• Sources of such interrupts are
• the keyboard,
• external memory (disk) drives,
• system clock.

33
The Interrupt Flag

• Occasionally, however, some processes are highly
  time-sensitive and do not tolerate any
  interrupts.
• The programmer can allow for this by turning off
  the ability to interrupt the CPU.
• This is done through the interrupt flag, IF.

34
The Interrupt Flag (cont.)

• When the IF is set, interrupts are enabled
  (normal value)
• When clear, interrupts are disabled.
• Instruction CLI clears the interrupt flag
  (disallows interrupts),
• Instruction STI sets the interrupt flag
  (re-enables interrupts).

35
Software Interrupts

• These are not really interrupts at all, but
  rather calls to outside routines such as BIOS or
  DOS functions.
• The INT instruction is used to introduce such
  interrupts.
• format as follows
• INT number

36
The INT Instruction

• Calls a routine type identified by a number.
• The number can range between 0 and FFh.
• The routine type can be one of several, and each
  type will have its own range of possible
  functions.
• The number indicated as an operand of the INT
  instruction then is referred to the Interrupt
  Vector Table (IVT).

37
The INT Instruction (cont.)

• INT maps all the interrupt numbers to operating
  system subroutine type.
• IVT located in the lowest 1KB of memory.
• Each entry is a 32-bit segment-offset address,
  which points to the location where the Operating
  System subroutine resides.
• Executes the routine at that address.

38
The INT Instruction (cont.)

• The various interrupt types are
• INT 10h Video services (cursor position,
  graphics, scroll)
• INT 16h Keyboard services (read keyboard and
  check status)
• INT 17h Printer services (initialize, print,
  return printer status).
• INT 1Ah Time of Day (no. of ticks since machine
  turned on)

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The INT Instruction (cont.)

• INT 1Ch User Timer Interrupt (executed 18.2
  times per sec.)
• INT 21h DOS services (DOS routines for file
  handling, memory management, I/O, - DOS function
  calls)
• The instruction IRET (interrupt return) tells the
  processor to resume execution at the next
  instruction of the calling program.

40
DOS Function Calls

• Called by INT 21h interrupt serv. routine
• Before the function type executes, it looks in
  register AH to determine the function number that
  identifies the subroutine itself (NOT the type of
  interrupt).
• Listed in page 116, figure 5.9.

41
DOS Function Calls

• These numbers are the ones that must be
  represented in register AH, as that is where the
  processor looks to see what the function number
  is.
• These are described in detail in the book.
• Please read over.

42
BIOS Level Video Control Instructions

• The BIOS level video control instructions are
  also listed in detail in the book.
• Please read them over.