Timeout for some demos

1. Timeout for some demos An instructive look at how the Linux Operating System sets up system data-structures

2. Simplified Block Diagram Central Processing Unit Main Memory system bus I/O device I/O device I/O device I/O device

3. CPU’s ‘fetch-execute’ cycle Fetch instruction at CS:EIP Advance EIP to next instruction Save context Get INTR ID Decode the fetched instruction Lookup ISR Execute the decoded instruction Execute ISR IRQ? yes IRET no

4. System registers and tables Interrupt Descriptor Table Global Descriptor Table GDTR IDTR EFER CR0 page frame (4-level) page-mapping tables CR2 CR4 CR3

5. ‘activity.s’ • This is a ‘boot-time’ application program • You assemble, link, install, and reboot • It runs entirely in x86 ‘real-mode’ • It maintains an array of counters • It ‘hooks’ all of the 256 interrupt-vectors • Every interrupt-occurrence gets counted • The counters are continuously displayed

6. ‘activity.cpp’ • This is a Linux application program • It’s similar to our previous demo ‘activity.s’ • It shows us all the interrupts which occur • But it runs in ‘protected-mode’ at ring3 • So it can’t modify the IDT directly • Instead it uses the services of an LKM • The LKM source-file is called ‘activity.c’

7. ‘mmake.cpp’ • This program makes it easy to compile an LKM for version 2.6 of the Linux kernel • If your LKM is named ‘myLKM.c’, then you compile it using: $ mmake myLKM • The result is a kernel object (‘myLKM.ko’) • You install the kernel object with ‘insmod’, and you remove it with ‘rmmod’, like this: $ /sbin/insmod myLKM.ko $ /sbin/rmmod myLKM.ko

8. Overview of the LKM user-space (ring3) kernel-space (ring0) Linux kernel int $0x80 standard runtime libraries iret ret call call ret activity activity.ko client server access barrier

9. Device-driver LKM • Our ‘dram.c’ module provides services • Lets applications ‘read’ physical memory • Lets applications find the memory’s size • Works with 32-bit Linux or 64-bit Linux • SysAdmin must setup device-file node

10. ‘showgdt.cpp’ • This application shows the Linux GDT • It uses services of the ‘dram.ko’ driver • You can try it in Kudlick classroom • You can try it on your ‘anchor’ machine

11. ‘showidt.cpp’ • This application shows the Linux IDT • It uses services of the ‘dram.ko’ driver • You can try running it on ‘anchor’ machine • You will see all 256 IDT gate-descriptors! • (But it’s designed only for 64-bit Linux)

12. pseudocode • We wrote an algorithm-description (using pseudocode) for UART’s THRE interrupt when you want to implement ‘redirection’ of console output to a remote terminal via the serial null-modem cable • We implemented our algorithm in a demo program (named ‘redirect.s’) which runs in 32-bit protected-mode

13. Two Interrupt-Controllers x86 CPU Serial-UART controller Master PIC (8259) Slave PIC (8259) INTR Keyboard controller Programmable Interval-Timer

14. A ‘rapid prototype’ tool • We wrote a ‘newapp64.cpp’ • It creates six pages of ‘boilerplate’ code for a boot-time assembly language application that will execute 64-bit code • It includes ‘isrGPF’ (for debugging aid) • We can use it to quickly create future demo programs that explore EM64T

15. In-class exercise • Use the ‘newapp64’ tool to create a ‘test’ program that you will install and execute on an ‘anchor’ machine – but first insert into the 64-bit portion of the code some type of invalid instruction which will trigger a General Protection Exception – so you can see what debugging information gets displayed by the ‘isrGPF’ fault-handler