Thursday, August 23, 2012

"Trap Return"'s esp to trap frame in xv6 and JOS

這二個OS在返回時,有一個共同目標。即是把esp 指定在trapframe的地方。但二者的implementation方式,只差在一行。

xv6是利用stack push pop的相對位置,效率高,但需小心過程中esp的修改(但其實這部份在gcc/assembler 的x86 calling convention已經幫coder做掉了,由ebp 這個reg 保存了call instruction之前esp的值。
       pushl %esp
       call trap
       addl $4, %esp

JOS是在trap return執行前 灌給esp 其保存的trap frame 位置。優點是在中間的stack 操作, 可以放心盡量使用,只要沒有破壞原本的trap frame 資料基本完整性即可 。但差在效能上的多執行一個function call的動作。 
void env_pop_tf (struct Trapframe *tf)
    __asm __volatile ("movl %0,%%esp\n" "\tpopal\n" "\tpopl %%es\n" "\tpopl %%ds\n" "\taddl $0x8,%%esp\n"   /* skip tf_trapno and tf_errcode */
                      "\tiret"::"g" (tf):"memory");
    panic ("iret failed");      /* mostly to placate the compiler */


Monday, August 20, 2012

[TALK] Memory Allocation In Classic Monolithic Kernel and Classic Micro Kernel

  •    I presented the topic "Memory Allocation in UNIX(classic monolithic kernel) and MINIX(classic micro kernel)" at Taiwan COSCUP2012 with 907 attendees-JuluOS Workshop.  I focus on memory allocation to show some differences between these 2 classic kernel.
  •    Something through this presentation I want to convey sincerely is that 
    1. History is the way to show wisdom about design. 
    2. Start to implement OS: Focus on the most important stuff you want, keep others to be as simple as possible.
    3. Join us to discuss OS, design OS, and implement OS.