MICROPROCESSOR AND ASSEMBLY LANGUAGE LECTURE11 BIOS AND DOS

MICROPROCESSOR AND ASSEMBLY LANGUAGE LECTURE-11 -BIOS AND DOS INTERRUPTS MUHAMMAD HAFEEZ DEPARTMENT OF COMPUTER SCIENCE GC UNIVERSITY LAHORE

TODAY’S AGENDA l l l Interrupts Mechanism Hardware Interrupts Software Interrupts

INTERRUPT l l l Interrupt is a mechanism through a program flow control can be altered When an interrupt occurs CPU suspends its execution of current program and transfer control to a special procedure, called Interrupt Service Routine (ISR) also called Handler When ISR is completed the control is transferred back to user program

INTERRUPTS VS. PROCEDURES Although analogous to procedures, there are few differences l Interrupt can be initiated by both software and hardware whereas procedures are initiated by software only l Procedures are used to carry out well defined requirements, interrupts are used to carry out unanticipated events such as to break from infinite loop Ctrl+break l ISR are either supplied with DOS or BIOS, procedures are loaded with user programs l ISR has number, procedure has name associated with it

CLASSIFICATION OF INTERRUPTS There are three main types of interrupts l Hardware Interrupts l Software Interrupts l Processor Interrupts or Exceptions

SOFTWARE INTERRUPTS Primarily generated by software l Initiated by execution of interrupt instruction INT in a program l Mainly used for accessing I/O devices such as keyboard, mouse, printer, screen or disk drives l l INT 21 H used to get a character or string from user, or display character or string on screen Two types of software interrupts l System Defined: l l Whose routines are provided by DOS or BIOS User Defined: l Whose routines are provided by user

SOFTWARE INTERRUPTS INT interrupt_type l Is the format to call ISR l Interrupt_type is a number ranging from 0 to 255 i. e. 256 different interrupt types, a large number l Each interrupt_type supports various parameterized services l There are more than 80 different services called functions provided by INT 21 H through DOS

SOFTWARE INTERRUPTS (BIOS, DOS AND USER INTERRUPTS) l From 256 following are s/w interrupts Interrupt Type Allocation 0 -1 Fh BIOS Interrupts 20 h-3 Fh DOS Interrupts 40 h-7 Fh Reserved 80 h-F 0 h ROM Basic F 1 h-FFh User Defined

HARDWARE INTERRUPTS l Generated by hardware devices to get CPU attention l l When a key is pressed, a hardware interrupt is generated for CPU to suspend its current operation and process key Hardware interrupts are of two types: l Maskable interrupts l l Generated by CPU pin INTR Non-Maskable interrupts l Generated by CPU pin NMI

HARDWARE INTERRUPTS l Non-Maskable interrupts l Serviced by CPU immediately after execution of current instruction l l Example: RAM parity error, indicating RAM malfuction Maskable interrupts l Can be delayed till some convenient time in the program l Controlled using interrupt flag

INTERRUPT FLAG (IF) l Interrupt flag controls whether maskable interrupt shall be delayed or not l l l IF=1, maskable interrupts are immediately serviced by CPU IF=0, maskable interrupts are delayed till IF=1 Instruction CLI clears I i. e. IF=0, STI set IF i. e. IF=1

IDENTIFYING HARDWARE INTERRUPTS l l When receive INTR interrupt the CPU initiates an interrupt acknowledgement sequence on its pin INTA Listening to this sequence interrupting device places its interrupt type on the data bus.

PROCESSOR INTERRUPTS OR EXCEPTIONS l Exceptions are processors interrupts to handle instruction faults l l Three types of exceptions depending on how they are reported and whether the instruction is started or not. l l l Divide by zero in an instruction Faults Traps Aborts Fault : is reported at instruction boundary. Divide by zero, or segment, system state is restored to before instruction state Trap: is reported after execution of instruction, such as overflow exception INT 4. system state is restored to after execution of instruction Aborts: severe error, hardware failures etc.

INTERRUPT PROCESSING l On start of PC, DOS and BIOS ISR are loaded into memory and stay there, called memory resident l l l To give a flexibility of storing these ISR anywhere in memory, their addresses are stored in a special table called Interrupt Descriptor Table (IDT) or Interrupt Vector Table (IVT) Since each interrupt hold address of an ISR consisting of 4 bytes (2 bytes for CS, 2 bytes for IP), total size of IVT is 1024 bytes In real mode IVT starts at address 0

INTERRUPT PROCESSING l Since each entry in IVT holds 4 bytes, interrupt_type multiplied by 4, the value can be used to track pointer to ISR in IVT l INT 0, the pointer at 0000 h, for INT 1, the pointer is at 0004 h, for INT 21 h, the pointer is at 0084 h. l For INT 21 h, IP shall be at byte number 00840085 h, and CS shall be at byte number 00860087 h

ILLUSTRATIONS

ILLUSTRATIONS

ILLUSTRATIONS

ILLUSTRATIONS

ILLUSTRATIONS

INTERRUPT PROCESSING Following actions occurs when an interrupt is executed l l l PUSH flag register on stack Clear Interrupt and Trap flag Push CS register onto stack Push IP register onto stack Load CS register from memory address [interrupt_type x 4 + 2] Load IP register from memory address [interrupt_type x 4]

INTERRUPT PROCESSING Procedures ends with RET instruction but ISR ends with IRET instruction, Following actions takes place when ISR completes l l l POP a word from TOS to IP POP a word from TOS to CS POP a word from TOS to flag register

BIOS INTERRUPTS Interrupts from 0 to 1 F are called BIOS interrupts, mainly because the routines used in these interrupts are stored in ROM of PC. l Interrupt Types 0 -7 l Interrupt types 0 -7 are reserved by Intel, with types 0 -4 being predefined. IBM uses type 5 for print screen. Types 6 and 7 are not used.

BIOS INTERRUPTS l l Interrupt 0 -- Divide Overflow l A type 0 interrupt is generated when a DIV or IDIV operation produces an overflow. This occurs when the quotient can not fit in the destination register. The interrupt 0 routine displays the message "DIVIDE OVERFLOW" and returns control to DOS. Interrupt 1 -- Single Step l Single-stepping is a useful debugging tool to observe the behavior of a program instruction by instruction. A type 1 interrupt is generated when the Trap Flag (TF) is set. The ISR for a type 1 interrupt can be used to display relevant information about the state of the program. For example, the contents of all registers can be displayed. To end single-stepping, the TF should be cleared. Note that there are dedicated instructions for setting and clearing the TF.

BIOS INTERRUPTS l l Interrupt 2 -- Non-Maskable Interrupt 2 is the hardware interrupt that cannot be masked out by clearing the TF. The IBM PC uses this interrupt to signal memory and I/O parity errors that indicate bad chips. Interrupt 3 -- Breakpoint The INT 3 instruction is the only single-byte interrupt instruction (opcode: CCh); other interrupt instructions are two-byte instructions. Inserting a breakpoint in a program involves replacing the program code byte by CCh while saving the program byte for later restoration to remove the breakpoint. Inserting breakpoints is used by program debuggers.

BIOS INTERRUPTS l l Interrupt 4 -- Overflow A type 4 interrupt is generated by the instruction INTO (interrupt if overflow) when the OF is set. Programmers may write their own routines to handle unexpected overflows. Note that executing the instruction INT 4 will invoke the ISR for this interrupt type unconditionally while INTO invokes it conditionally on the OF. INTO is not used normally as the overflow condition is usually detected and processed using the instructions JO and JNO. Interrupt 5 -- Print Screen The BIOS interrupt 5 routine sends the video screen information to the printer. An INT 5 instruction is generated by the keyboard interrupt routine (interrupt type 9) when the Prt. SC (print screen) key is pressed.

BIOS INTERRUPTS l l Interrupt Types 8 h-Fh The 8086 has only one pin, INTR pin, for maskable hardware interrupt signals. To allow more devices to interrupt the 8086, IBM uses an interrupt controller, the Intel 8259 Programmable Interrupt Controller chip, which can interface up to eight devices. Interrupt types 8 h-Fh are generated by hardware devices connected to the 8259 chip. The original version of the PC uses only interrupts 8, 9, and Eh. Interrupt 8 -- Timer The IBM PC contains a timer circuit that generates an interrupt once every 54. 92 milliseconds (about 18. 2 times per second). The BIOS interrupt 8 routine services the timer circuit. It uses the timer signals (ticks) to keep track of the time of the day.

BIOS INTERRUPTS l l Interrupt 9 -- Keyboard The interrupt 9 is generated by the keyboard each time a key is pressed or released. The BIOS interrupt 9 routine reads a scan code and stores it in the keyboard buffer. In addition, it also identifies special key combinations such ctrl-break. The keyboard buffer has the capacity to store up to 15 keys. When the buffer is full, pressing a key causes the BIOS to beep, indicating that the key stroke is lost. The keyboard controller supplies the key identity by means of a scan code. The scan code of a key is simply an identification number given to the key based on its location in the keyboard. The scan code has no relation to the ASCII code. The interrupt 9 routine receives the scan code and generates the equivalent ASCII code, if there is one. Both the scan code and the ASCII code are placed in the keyboard buffer. Interrupt E -- Diskette Error The BIOS interrupt Eh handles diskette errors.

BIOS INTERRUPTS l l Interrupt Types 10 h-1 Fh The interrupt routines 10 h-1 Fh can be called by application programs to perform various I/O operations and status checking. Interrupt 10 h -- Video The BIOS interrupt 10 h routine is the video driver. Associated with each I/O device. There is a device controller or I/O controller that acts as a hardware interface between the processor and the I/O device. The device controller performs many of the low-level tasks specific to the I/O device. This allows the CPU to interact with the device at higher level. For each device controller, there is a software interface that provides a clean interface to access the device. This software interface is called the device driver.

BIOS INTERRUPTS l l Interrupt 11 h -- Equipment Check The BIOS interrupt 11 h routine returns the equipment configuration of the particular PC. The return code is placed in register AX. Interrupt 12 h -- Memory Size The BIOS interrupt 12 h routine returns in register AX the amount of conventional memory a computer has. Conventional memory refers to memory circuits with addresses below 640 KByte. The unit for the return value is in Kbytes.

BIOS INTERRUPTS l l l l Interrupt 13 h -- Disk I/O The BIOS interrupt 13 h routine is the disk driver. It allows application programs to do disk I/O. Interrupt 14 h -- Communications The BIOS interrupt 14 h routine is the communications driver that interacts with the serial ports. Interrupt 15 h -- Cassette This interrupt was used by the original PC for cassette interface. Interrupt 16 h -- Keyboard I/O The BIOS interrupt 16 h routine is the keyboard driver. BIOS provides keyboard service routines under INT 16 H.

BIOS INTERRUPTS l l l l l Interrupt 17 h -- Printer I/O The BIOS interrupt 17 h routine is the printer driver. Interrupt 18 h -- BASIC The BIOS interrupt 18 h routine transfers control to ROM BASIC. Interrupt 19 h -- Bootstrap The BIOS interrupt 19 h routine reboots the system. Interrupt 1 Ah -- Time of Day The BIOS interrupt 1 Ah routine allows a program to get and set the timer tick count. Interrupt 1 Bh -- Ctrl Break This interrupt is called by the INT 9 routine when the Ctrl-break key is pressed. The BIOS interrupt 1 Bh routine contains only an IRET instruction. Users may write their own routine to handle the Ctrl-break key.

BIOS INTERRUPTS l l Interrupt 1 Ch -- Timer Tick Interrupt 1 Ch is called by the INT 8 routine each time the timer circuit interrupts. The BIOS interrupt 1 Ch routine contains only an IRET instruction. Users may write their own service routine to perform timing operations. Interrupts 1 Dh-1 Fh These interrupt vectors point to data instead of instructions. The interrupt 1 Dh, 1 Eh, and 1 Fh point to video initialization parameters, diskette parameters, and video graphics characters, respectively.

DOS INTERRUPTS l The interrupt types 20 h-3 Fh are serviced by DOS routines that provide high-level service to hardware as well as system resources such as files and directories. The most useful is INT 21 H, which provides many functions for doing keyboard, video, and file operations.

DOS INTERRUPTS l l Interrupt 20 h -- Program Terminate Interrupt 20 h can be used by a program to return control to DOS. But because CS must be set to the program segment prefix before using INT 20 h, it is more convenient to exit a program with INT 21 H, function 4 Ch. Interrupt 21 h -- Function Request This routine provides over 80 functions that maybe classified as character I/O, file access, memory management, disk access, networking, and miscellaneous.

INT 21 H FUNCTIONS