EE314 Microprocessor Systems Chapter 5 Interrupt processing Objectives: The difference between hardware and software interrupts The difference between maskable and nonmaskable interrupts Interrupt processing procedures The vector address table Multiple interrupts and interrupt priorities Special function interrupts The general requirement of all interrupt handlers Based on "An Introduction to the Intel Family of Microprocessors" by James L. Antonakos

5.2 Hardware and Software Interrupts The nonmaskable interrupt is generated by en external device, trough a rising edge on the NMI pin. Cannot be ignored by the microprocessor. Generates a Type 2 interrupt (address 0008H in the Interrupt vector table) The maskable interrupts (0…FFH) can be generated by: an external device, trough a high logic level on the INTR pin (the external device has to specify the interrupt number). Hardware interrupts (IF (interrupt flag) in FLAGS register enables or disables (masks) the P to accept maskable interrupts.) microprocessor itself (i.e. when trying to divide by 0), (the interrupt number is hardware defined). Software interrupts (exceptions) using the INT instruction (followed by the interrupt number (type)). Interrupt priority Divide-error Highest INT, INTO NMI INTR Single-step Lowest

5.3 The Interrupt Vector Table (or Interrupt Pointer Table) The memory block from address 00000 to 003FF. There are 1024 bytes, each of the 256 maskable interrupts uses four bytes to store the address where the corresponding ISR (Interrupt Service Routine) begins. The ISR address for interrupt number xx is stored beginning at address xx*4, in form CS:IP. From low to high address, the bytes are stored in the order: IP low, IP high, CS low and CS high (byte swapping). Consequences: After RESET the P cannot begin running from physical address 00000. The first instruction is fetched at address FFFF0H. Before using an interrupt, its corresponding ISR address has to be stored in the interrupt vector table. ISRs are handled as FAR routines (both CS and IP specified). Vectors 0 to 18 are predefined, 19 to 31 are reserved by Intel, 32 to 255 are unassigned (free to use):

5.4 The Interrupt Processing Sequence Hardware INTR Internal HW Int. Software Interrupts no yes Interrupt not accepted IF=1 IF=1 yes no Inform external devices that an interrupt acknowledge cycle began first INTA cycle (8086 - INTA pin = low) (Pentium - M/IO,D/C,W/R,ADS = 0) Interrupt not accepted The interrupt type is the operand NMI External devices requesting an interrupt transmits trough data bus the interrupt type The interrupt type is predefined second INTA cycle read interrupt type on Data Bus Save processor information on stack: - FLAGS register - Return address = CS:IP Fetch the address of the ISR: - CS:IP from Interrupt Vector Table at address 4*(interrupt type) Clear IF and TF (no further maskable interrupts allowed) ISR execution

Return to interrupted program 5.5 Multiple Interrupts ISR execution (non NMI) NMI ISR execution IRET no yes NMI Execution of all instructions no NMI Execution of 1 instruction The interrupt type is predefined Load processor information from stack: - Return address = CS:IP - FLAGS register Return to interrupted program IRET Load processor information from stack: - Return address = CS:IP - FLAGS register Save processor information on stack: - FLAGS register - Return address = CS:IP Clear IF and TF (no further maskable interrupts allowed) Fetch the address of the NMI ISR

5.6 Special Interrupts generates a type 0 interrupt. 0400:1100 B3 00 MOV BL,0 0400:1102 F6 F3 DIV BL 0400:1104 …. Divide Error: type 0, hardware generated by the P when quotient doesn’t fit in destination (division by 0) return address Single step: type 1, hardware generated by the P (if TF=1) after each instruction. After pushing flags onto stack, TF is cleared (IF also), so ISR itself is not interrupted. Returning after ISR, the flags are restored, another interrupt is generated after next instruction. A program example to set or reset the TF: PUSHF POP AX OR AX,100H PUSH AX POPF moves FLAGS to AX PUSHF POP AX OR AX,100H PUSH AX POPF updates TF moves AX to FLAGS Replaced by INT 3 code (CCH) by setting a breakpoint. NMI: type 2, hardware generated by an external device on emergent events (i.e. power fail). Rising edge active. 0400:1100 3C 00 CMP AL,0 0400:1102 75 01 JNZ XYZ 0400:1104 EE OUT DX,AL 0040:1105 FE C0 XYZ: INC AL Breakpoint: type 3, software generated by a single-byte instruction, INT 3. Overflow: type 4, generated by INTO instruction if OF=1. Interrupt request has to stay active until acknowledged: External maskable interrups: type 0-255 via INTR pin P generates two pulses on INTA pin. During the second pulse, the external device has to put on data bus (D0…D7) the interrupt type.

5.6 Special Interrupts Interrupts may occur in unexpected moments during main program execution (i.e. between setting of a flag as result of an arithmetical instruction and the subsequent conditional jump hanging on the flag value). After returning from ISR, the main program has to continue undisturbed by changes made in P’s internal state (environment or context): flags, registers. An ISR can perform multiple functions hanging on the value of an input parameter (i.e. the value in the AH register). Before occurrence of the interrupt (usualy a software one) the value of the parameter is prepared. The corresponding ISR tests the parameter and perform the action required by its value. The interrupt acknowledge mechanism saves FLAGS and return address, but no register content. The Interrupt Service Routine (ISR) is responsible for saving all the used register’s value on stack (PUSH), and to recover it (POP) before returning. MYISR: PUSHA … POPA IRET Usually, all registers are saved (PUSHA) and recovered (POPA)

5.6 Special Interrupts A simple circuit able to place an 8-bit interrupt number (type) onto data bus second active pulse of INTA first active pulse of INTA P is reading the byte on AD7…AD0 INTR can deactivate after activation of INTA Interrupt request ... INTR AD7 . . 8088 AD0 INTA INTR INTA AD7 AD6 AD5 AD4 AD3 AD2 AD1 AD0 1 +5V 8 x 4.7K 74LS 244 (octal buffer) G1 G2 The octal buffer outputs are three-state Data BUS is free for carrying data between P and other devices in system The octal buffer controls the Data BUS.

5.6 Special Interrupts A simple prioritized interrupt circuitry P is reading the byte on AD7…AD0 INT 0D0H requested A simple prioritized interrupt circuitry second active pulse of INTA first active pulse of INTA INTA deactivates INTR P interrupt request INT2 INTR INTA AD7 AD6 AD5 AD4 AD3 AD2 AD1 AD0 ... INTR AD7 . . 8088 AD0 INTA 1 +5V 4.7K INT2 request 74LS 374 (octal flip-flop) OE INT0 INT2 . INT7 (priority encoder) E1 GS +5V 4.7K D Q CLR The octal buffer outputs are three-state Data BUS is free for carrying data between P and other devices in system The octal buffer controls the Data BUS.

5.7 Interrupt Service Routines Simple example: one second time interval generator using a 60Hz signal on NMI ; ISR for NMI NMITIME: DEC COUNT ;decrement 60th’s counter, (COUNT)(COUNT)-1 ;(ZF)1 if (COUT)=0 JNZ EXIT ;did we go to 0?, jump only if (ZF)=0 MOV COUNT, 60 ;yes, reload the counter, (COUNT)3Ch CALL FAR PTR ONESEC ;call ONESEC, [SP-1],[SP-2] (CS), ;[SP-3],[SP-4]EXIT,(SP)(SP)-4 ;reverse action when return from ONESEC EXIT: IRET ;(IP)[SP],[SP+1], (CS)[SP+2],[SP+3], ;(FLAGS)[SP+4],[SP+5], (SP)(SP)+6, ;reverse action to what ;happened accepting NMI ; main program slide preparing the action of NMI’s ISR MOV COUNT, 60 ;init 60th’s counter , (COUNT)3Ch PUSH DS ;save current DS content , [SP-1],[SP-2] (DS),(SP) (SP)-2 SUB AX, AX ;set new DS content to 0000, (AX) 0 MOV DS, AX ;(DS) 0 LEA AX, NMITIME ;load address of NMITIME ISR, (AX) NMITIME MOV [8], AX ;store IP address in IPT, replacing regular NMI’s ISR address ;[8],[9] NMITIME MOV AX, CS ;store CS address in IPT, (AX) (CS)=current code segment MOV [0AH], AX ;[0Ah],[0Bh] (CS) POP DS ;get old DS content back, (DS)  [SP],[SP+1], (SP) (SP)+2

5.7 Interrupt Service Routines Simple example: A Divide-Error Handler If a divide error occurs, the ISR will load AX wit 101h, and DX with 0. A error message will be displayed. The error message is in DATA segment beginning at address DIVMSG and ends wit a “$” character. The DISPMSG procedure (subroutine) (not shown) displays the character string found in DATA segment until the first “$” character. ISR address has to be loaded (not shown) at address 0000 in the IPT (INT 0). ; preparing the error message in DATA segment .DATA DIVMSG DB ’Division by zero attempted!$’ ;first character, ”D”, at address DIVMSG in DATA segment ; ISR for Divide-Error DIVERR: PUSH SI ;save current SI content , [SP-1],[SP-2] (SI),(SP) (SP)-2 MOV AX, 101h ;load result with default, (AX) 101h SUB DX, DX ;clear DX, (DX) 0 LEA SI, DIVMSG ;init pointer to error message, (SI) DIVMSG ;(passing parameter trough register) CALL FAR PTR DISPMSG ;output error message, [SP-1],[SP-2] (CS), ;[SP-3],[SP-4]return address,(SP)(SP)-4 ;reverse action when return from DISPMSG POP SI ;get old SI content back, (SI)  [SP],[SP+1], (SP) (SP)+2 IRET ;(IP)[SP],[SP+1], (CS)[SP+2],[SP+3] =return address = ;(the address of the first instruction ;after the DIV generating the error) ;(FLAGS)[SP+4],[SP+5], (SP)(SP)+6, ;reverse action to what happened accepting INT 0

5.7 Interrupt Service Routines Simple example: An ISR with Multiple Functions ; ISR for INT 20H ISR20H: CMP AH, 4 ;AH must be 0-3 only ;(?)(AH)-4, (ZF)1 if (?)=0, (CF)1 if (?)<0 (unsigned) ;(OF)1 if (?)<-128 or (?)>127, ;(PF)1 if (?) contains an even number of “1”s, ;(AF) if a transport from bit 3 to bit 4 occurred, ;(SF) if (?)<0 (signed) JNC EXIT ;AH >3, ISR returns without any effect CMP AH, 0 ;AH = 0 ?, (?)(AH)-0, (ZF)1 if (?)=0, ... JZ ADDAB ;AH = 0, jump to add function CMP AH, 1 ;AH = 1 ?, (?)(AH)-1, (ZF)1 if (?)=0, ... JZ SUBAB ;AH = 1, jump to subtract function CMP AH, 2 ;AH = 2 ?, (?)(AH)-4, (ZF)1 if (?)=0, ... JZ MULAB ;AH = 2, jump to multiply function DIVAB: DIV BL ;AH = 3, use divide function IRET ADDAB: ADD AL, BL ;add function SUBAB: SUB AL, BL ;subtraction function MULAB: MUL BL ;multiply function ; main program has to store the address of INT 20’s ISR at address 80h in IPT.