Lecture 016

Registers

• General purpose registers: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp, %esp

  • %eax return value register
  • %esp stack pointer register
  • %ebp base pointer register (used for stack frame)

• Segment registers: %cs (code), %ss (stack), %ds (data), %es (extra), %fs (extra), %gs (extra), important during context switch

• Instruction pointer: %eip (can't read or write directly)

• EFLAGS register: condition code flags, control flags, system flags (if set, expect certain value in regular registers, and some for arithmetic), generally don't write it directly

• Control registers: %cr0, %cr2, %cr3, %cr4

  • %cr0 control flags: paging enable, protected mode enable, etc.
  • %cr1 reserved -> don't touch, don't look
  • %cr2 contains address that last caused page fault
  • %cr3 page directory base register (PDBR), contain address of current page directory
  • %cr4 control flags: protected mode extensions, virtual-8086 mode extensions, etc.
  • You need to movl with PL0

• esp0 (NOT A REGISTER) hardware "task segment": default stack pointer when trap into kernel, set by OS in kernel mode

Hardware hash "Task" built in, but:

• we might not need all features of hardware task, implement software task can be more efficient and flexible

• people expect OS to run on any hardware

• but to use paging, we need "protected mode" running hardware task (we will just have one task)

disable_interrupt does not disable traps, it "defer" interrupts. In a multi-processor system, it only disable for current CPU, but not all CPUs.

Program Loader:

• ELF file is complicated, that's what we need to load

• load code, rodata, data, bss, stack