CS452 - Real-Time Programming - Spring 2008

Lecture 9 - Kernel Code

Questions & Comments

1. Hand back the first assignment, with comments.
   • PSW
   • TD of active
   • interrupts
2. Trains lab

Hardware Facts for Eos

Memory Map

• 0x000000 to 0x09FFFF: low memory (size is 640K)
• 0x0A0000 to 0x0FFFFF: VGA, etc (size is 360K)
• 0x100000 to 0x1XXXXF: your kernel (size is determined by you)
• (Ox1XXXXF + 0x1) to 0x1FFFFF: free memory for tasks (size is the remainder of 2 Mbyte)

Devices

Accessed by special I/O instructions

Architecture of X86

General Purpose Registers

• E[A-D]X: general purpose registers
• ESP: the stack pointer
• EBP: the base pointer
• ESI, EDI: ??

All these registers have special names for the low 16 bits. (drop the E), and for the low 8 bits (add an L), and for the second 8 bits (add an H)

Addressing Memory

Physical address is

• start of segment + offset

Helps to provide

1. position independent (relocatable) code
2. memory protection
   • program owns segment(s)
   • segment has low and high limits
   • program can't access outside the limits

Six segment registers

1. CS
2. DS
3. ES
4. FS
5. GS
6. SS

Each segment register is an index into a Global Descriptor Table (GDT), which contains

1. low limit
2. high limit
3. etc.

Index is actually contents of register >> 3.

Setting up a Task

Every task requires at least two GDTs

• accessed through CS and DS
• This includes the kernel

The compiler assumes that DS = SS

• That is, the stack is in the data segment.

Set DS = ES = FS = GS = SS for each task

There is no GDT in place when the kernel boots

• You have to create them

How is this done

1. The address of the table is kept in the special register GDTR
2. lgdt sets GDTR; sgdt reads GDTR

Yet another register EFLAGS contains processor state such as

• condition codes
• what is enabled
• Getting started you want to have interrupts disabled!

Setting up a tas

• CS points to code in ELF executable
• DS points to memory you allocate for the task.

Context Switch in the 386

int <vector>

• pushes EFLAGS, CS, EIP onto the active task's stack
• finds entry <vector> in the interrupt descriptor table (IDT)
• jumps to the address in the IDT[<vector>]

iretl

• pops EFLAGS, CS, EIP from the active task's stack
• puts them in the registers

Task to kernel

1. int <vector> in task
2. pushal saves all the active task's registers on the active task's stack
3. switch to kernel stack
   1. you got CS, EIP from IDT
   2. ESP from your magic place
   3. DS from the kernel variables
4. restore remainder of kernel state from stack

Kernel to task

1. save kernel state on kernel stack
2. switch to active task's stack
3. restore general purpose registers: popal
4. iretl gets EFLAGS, CS, EIP from the task stack.

To get a task going

1. Set up its stack so that it's all ready to run its first instruction
2. Schedule
3. Kernel to task

For the first user task there is only one possibility for active.

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