Reverse-Engineering the Interrupt Circuitry in the Intel 8086 Processor
owl writes:
http://www.righto.com/2023/02/8086-interrupt.html
Interrupts have been an important part of computers since the mid-1950s,1 providing a mechanism to interrupt a program's execution. Interrupts allows the computer to handle time-critical tasks such as I/O device operations. In this blog post, I look at the interrupt features in the Intel 8086 (1978) and how they are implemented in silicon, a combination of interesting circuitry and microcode.
[...] The idea behind an interrupt is to stop the current flow of execution, run an interrupt handler to perform a task, and then continue execution where it left off. An interrupt is like a subroutine call in some ways; it pushes the current segment register and program counter on the stack and continues at a new address. However, there are a few important differences. First, the address of the interrupt handler is obtained indirectly, through an interrupt vector table. Interrupts are numbered 0 through 255, and each interrupt has an entry in the vector table that gives the address of the code to handle the interrupt. Second, an interrupt pushes the processor flags to the stack, so they can be restored after the interrupt. Finally, an interrupt clears the interrupt and trap flags, blocking more interrupts while handling the interrupt.
The 8086 provides several types of interrupts, some generated by hardware and some generated by software. For 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.2 Typically, most interrupts use the INTR pin, signaling things such as a timer, keyboard request, real-time clock, or a disk needing service. 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. The 8086 also has a RESET pin that resets the CPU. Although not technically an interrupt, the RESET action has many features in common with interrupts, so I'll discuss it here.
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