1/13/2024 0 Comments Stack the states for pcEach of the 256 interrupt types has an entry holding The diagram below shows how the vector table is implemented. To support single-step mode in debuggers, the Trap flag generate an interrupt on every instruction. The INTO instruction (Interrupt if Overflow) generates an interrupt The divide instructions generate an interrupt if a divide-by-zero or overflow occurs. The one-byte INT 3 instruction creates a breakpoint interrupt for debugging. These functions could be everything from a floppy disk operation to accessing the printer. These software interrupts were used in the IBM PC to execute a function in the BIOS, the layer underneath the operating system. The INT n instruction creates an interrupt of the specified type (0 to 255). On the software side, the 8086 has multiple types of interrupts generated by different instructions. The 8086 also has a RESET pin that resets the CPU.Īlthough not technically an interrupt, the RESET action has many features in common with interrupts, so I'll discuss it here. The NMI interrupt is designed for things such as parity error or an impending power failure, which are so critical they can't be delayed. Typically, most interrupts use the INTR pin, signaling things such as a timer, keyboard request, real-time clock, The 8086 provides several types of interrupts, some generated by hardware and some generated by software.įor hardware interrupts, the INTR pin on the chip generates a maskable interrupt when activated, while the NMI pin on the chip generates a higher-priority non-maskable interrupt. Second, an interrupt pushes the processor flags to the stack, so they can be restored after the interrupt.įinally, an interrupt clears the interrupt and trap flags, blocking more interrupts while handling the Interrupts are numbered 0 through 255, and each interrupt has an entry in the vector table that gives the address However, there are a few important differences.įirst, the address of the interrupt handler is obtained indirectly, through an interrupt vector table. The stack and continues at a new address. ![]() The idea behind an interrupt is to stop the current flow of execution, run an interrupt handler to performĪ task, and then continue execution where it left off.Īn interrupt is like a subroutine call in some ways it pushes the current segment register and program counter on Click on this image (or any other) for a larger version. This photo shows the chip's single metal layer the polysilicon and silicon are underneath. ![]() The 8086 die under a microscope, with main functional blocks labeled. The BIU handles bus activity, while the Execution Unit (EU) executes instructions and microcode.īoth parts are extensively involved in interrupt handling. I've labeled the key functional blocks the ones that are important to this discussion are darker and will be discussed in detail below.Īrchitecturally, the chip is partitioned into a Bus Interface Unit (BIU) at the top and an Execution Unit (EU) below. The metal layer on top of the chip is visible, with the silicon and polysilicon mostly hidden underneath.Īround the edges of the die, bond wires connect pads to the chip's 40 external pins The die photo below shows the chip under a microscope. I've been reverse-engineering the 8086 starting with the silicon die. Silicon, a combination of interesting circuitry and microcode. In this blog post, I look at the interrupt features in the Intel 8086 (1978) and how they are implemented in Interrupts allows the computer to handle time-critical tasks such as I/O device operations. Providing a mechanism to interrupt a program's execution. ![]() Interrupts have been an important part of computers since the mid-1950s, 1
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