Assembly 08 Interrupts Introduction Interrupts are similar to

Assembly 08 Interrupts

Introduction • Interrupts are similar to procedures – They are used to alter a program’s control flow – The interrupt service is also preformed using a routine. • They are different – The are not invoked by a call instruction. – Software interrupts. • Software invoked interrupts are caused by executing the int instruction. – Hardware interrupts • These interrupts handle an un-anticipated event, which causes are external to the program. • User caused interrupts (Ctrl-C) • Hardware failure or hardware caused event. – Exceptions, such as divide by zero

Interrupt Service • The raise of an interrupt, usually called for a service/help. • This service is performed using what is calls an interrupt service routine (ISR) or handler. • When the ISR is completed, – The interrupted program resumes execution as if it were not interrupted. – The interrupted routine my decide to terminate the program and not to return.

Interrupt Classes

Software interrupts • Invoking software interrupts is performed by the int instruction. • Software interrupts are mainly used to access I/O devices such as a keyboard, printer, display screen, disk drive, etc. • Software interrupts are also classified into – system-defined – user-defined.

Hardware interrupts • Hardware interrupts are generated by hardware devices to get the attention of the processor. • Nonmaskable interrupts is always handled by processor – Example: the RAM parity error indicating memory malfunction. • Maskable interrupts can be delayed until execution reaches a convenient point. – Example: while running a ISR.

Protected Mode’s Interrupts • Interrupts are identified by a type number, called a vector. • Pentium supports 256 different interrupt types (0. . 255). • The interrupt type number/vector, is used as an index into a table that stores the addresses of ISRs. • This table is called the interrupt descriptor table (IDT). • Each descriptor is a pointer to an ISR and requires eight bytes. • The interrupt type number is scaled by 8 to form an index into the IDT. • The IDT may reside anywhere in physical memory. • The location of the IDT is maintained in an IDT register IDTR. • The IDTR is a 48 -bit register that stores the 32 -bit IDT base address • and a 16 -bit IDT limit value.

The IDT

IDT continue • The IDT can have three types of descriptors: – Interrupt gate – Trap gate – Task gate. • Interrupt and Task gates include – – 16 -bit segment selector, 32 -bit offset, Descriptor privilege level (DPL), P bit to indicate whether the segment is present or not.

Interrupt Gates

Interrupt Handling • When an interrupt occurs – The segment selector is used to select a segment descriptor that is in either the GDT or the current LDT (based on the TI bit). – The segment descriptor provides the base address of segment that contains the interrupt service routine and the offset part comes from the interrupt gate. – Start the ISR by • • Push the EFLAGS register onto the stack; Clear the interrupt and trap flags; Push CS and EIP registers onto the stack; Load CS with the 16 -bit segment selector from the interrupt gate; • Load EIP with the 32 -bit offset values from the interrupt gate.

Protected-mode interrupt invocation.

Interrupt and Trap gates • Processing a trap gate is similar interrupt gate except it does not modify the (interrupt flag) IF flag. • Some types of exceptions also push an error code onto the stack. • The exception handler can use this error code in identifying the cause for the exception.

Exceptions • Exceptions could be faults, traps, or aborts depending on the way they are reported and handled • Examples – Divide by zero – Segment not present – Breakpoint interrupt (debugger)

File System Calls-Software Interrupt • System call 8 — Create and open a file Inputs: EAX = 8 EBX = file name ECX = file permissions Returns: EAX = file descriptor Error: EAX = error code • System call 5—Open a file Inputs: EAX = 5 EBX = file name ECX = file access mode EDX = file permissions Returns: EAX = file descriptor Error: EAX = error code

File System Calls-Software Interrupt • System call 3 — Read from a file Inputs: EAX = 3 EBX = file descriptor ECX = pointer to input buffer EDX = buffer size (maximum number of bytes to read) Returns: EAX = number of bytes read Error: EAX = error code • System call 4 — Write to a file Inputs: EAX = 4 EBX = file descriptor ECX = pointer to output buffer EDX = buffer size (number bytes to write) Returns: EAX = number of bytes written Error: EAX = error code

File System Calls-Software Interrupt • System call 6 — Close a file Inputs: EAX = 6 EBX = file descriptor Returns: EAX = — Error: EAX = error code • System call 19— lseek (Updates file pointer) Inputs: EAX = 19 EBX = file descriptor ECX = offset EDX = whence Returns: EAX = byte offset from the beginning of file Error: EAX = error code

Examples ; ------------------------------; Put character procedure receives the character in AL. ; ------------------------------putch: pusha mov [temp_char], AL mov EAX, 4 ; 4 = write mov EBX, 1 ; 1 = std output (display) mov ECX, temp_char ; pointer to char buffer mov EDX, 1 ; # bytes = 1 int 0 x 80 popa ret

Examples ; ------------------------------; Get string procedure receives input buffer pointer in EDI ; and the buffer size in ESI. ; ------------------------------getstr: pusha pushf mov EAX, 3 ; file read service mov EBX, 0 ; 0 = std input (keyboard) mov ECX, EDI ; pointer to input buffer mov EDX, ESI ; input buffer size int 0 x 80 dec EAX done_getstr: mov byte[EDI+EAX], 0 ; append NULL character popf popa ret

Example-A file copy program %include "io. mac" %define BUF_SIZE 256. DATA in_fn_prompt db ’Please enter the input file name: ’, 0 out_fn_prompt db ’Please enter the output file name: ’, 0 in_file_err_msg db ’Input file open error. ’, 0 out_file_err_msg db ’Cannot create output file. ’, 0. UDATA in_file_name resb 30 out_file_name resb 30 fd_in resd 1 fd_out resd 1 in_buf resb BUF_SIZE. CODE. STARTUP Put. Str in_fn_prompt ; request input file name Get. Str in_file_name, 30 ; read input file name Put. Str out_fn_prompt ; request output file name Get. Str out_file_name, 30 ; read output file name

Example-A file copy program ; open the input file mov EAX, 5 ; file open mov EBX, in_file_name ; pointer to input file name mov ECX, 0 ; file access bits (0 = read only) mov EDX, 0700 ; file permissions int 0 x 80 mov [fd_in], EAX ; store fd for use in read routine cmp EAX, 0 ; open error if fd < 0 jge create_file Put. Str in_file_err_msg newline jmp done create_file: ; create output file mov EAX, 8 ; file create mov EBX, out_file_name; pointer to output file name mov ECX, 0700 ; read/write/exe by owner only int 0 x 80 mov [fd_out], EAX ; store fd for use in write routine

Example-A file copy program cmp EAX, 0 ; create error if fd < 0 jge repeat_read Put. Str out_file_err_msg newline jmp close_exit ; close the input file & exit repeat_read: ; read input file mov EAX, 3 ; file read mov EBX, [fd_in] ; file descriptor mov ECX, in_buf ; input buffer mov EDX, BUF_SIZE ; size int 0 x 80 ; write to output file mov EDX, EAX ; byte count mov EAX, 4 ; file write mov EBX, [fd_out] ; file descriptor mov ECX, in_buf ; input buffer int 0 x 80

Example-A file copy program cmp EDX, BUF_SIZE ; EDX = # bytes read jl copy_done ; EDX < BUF_SIZE ; indicates end-of-file jmp repeat_read copy_done: mov EAX, 6 ; close output file mov EBX, [fd_out] int 0 x 80 close_exit: mov EAX, 6 ; close input file mov EBX, [fd_in] int 0 x 80 done: . EXIT

Real-Mode Interrupts • DOS and BIOS provide several software interrupt services. • I/O devices can be accessed in three ways. – DOS and BIOS provide two ways of interacting with the system I/O devices. – The third method involves direct I/O access. This method is low level in nature and more complicated than the high-level access provided by DOS and BIOS. • Direct access of I/O devices is supported by in and out instructions.

Interrupt Processing • Upon the occurrence of an interrupt occurs, the following are performed: – Push flags register onto the stack; – Clear interrupt and trap flags to disable further interrupts; – Push CS and IP registers onto the stack; – Load CS with the 16 -bit data at memory address (interrupt-type * 4 + 2); – Load IP with the 16 -bit data at memory address (interrupt-type * 4).

Interrupt Processing • on iret instruction, the following are performed: – Pop the 16 -bit value on top of the stack into IP register; – Pop the 16 -bit value on top of the stack into CS register; – Pop the 16 -bit value on top of the stack into the flags register. • A typical ISR structure is shown below. <save the registers used in the ISR> sti ; enable further interrupts. . . <ISR body>. . . <restore the saved registers> iret ; return to the interrupted program

Real-mode interrupt vector table.

Software Interrupts

Dos and Bios Int • Both DOS and BIOS provide several interrupt service routines to access I/O devices. • DOS services are provided by int 21 H. • DOS provides more than 80 different services (called functions). • The interrupt services provided by DOS and BIOS are not mutually exclusive. • Bios keyboard services use int 16 H

A string read code STR_LENGTH EQU 81 %include "io. mac". STACK 100 H. DATA prompt_msg 1 db "Please enter maximum string length: ", 0 prompt_msg 2 db "Please enter a string: ", 0 string_msg db "The string entered is: ", 0 error_msg db "No string read. Buffer size must be at least 1. ", 0. UDATA temp_buf resb STR_LENGTH+2 in_string resb STR_LENGTH. CODE. STARTUP Put. Str prompt_msg 1 Get. Int CX ; max. string length Put. Str prompt_msg 2 mov BX, in_string ; BX = pinter to input buffer call read_string ; to call read_string procedure Put. Str string_msg Put. Str in_string 32: . EXIT

A string read code ; Get string (of maximum length 80) from keyboard. ; BX <-- pointer to a buffer to store the input string ; CX <-- buffer size = string length + 1 for NULL ; If CX <2, reports error and terminates. ; If CX > 81, CX = 81 is used to read at most 80 characters. ; -----------------------------read_string: pusha ; ES = DS for use by the string instruction--movsb mov DX, DS mov ES, DX mov DI, BX ; DI = buffer pointer inc CX ; space for NULL ; check CX value cmp CX, 2 jl bailout cmp CX, 81 jle read_str mov CX, 81 read_str:

A string read code ; use temporary buffer temp_buf to read the string ; using functin 0 AH of int 21 H mov DX, temp_buf mov SI, DX mov [SI], CL ; first byte = # chars. to read mov AH, 0 AH int 21 H inc SI ; second byte = # chars. read mov CL, [SI] ; CX = # bytes to copy inc SI ; SI = input string first char. cld ; forward direction for copy rep movsb mov byte[DI], 0 ; append NULL jmp done bailout: Put. Str error_msg done: popa ret