Questions

Questions

• Why does BIOS read IPL to 0x7c00? Does current architecture use this mechanism?
• What is OS header defined in os.ls? What is the role of it?

os.ls

OUTPUT_FORMAT("binary");

SECTIONS {
  .head 0x0 : {
    LONG(64 * 1024) /* 0 : size(stack+.data+heap) */
      LONG(0x69726148) /* 4 : "Hari" */
      LONG(0) /* 8 : mmarea*/
      LONG(0x310000) /* 12 : stack初期値 & .data転送先 */
      LONG(SIZEOF(.data)) /* 16 : size of .data */
      LONG(LOADADDR(.data)) /* 20 : size of .data */
      LONG(0xE9000000) /* 24 : E9000000 */
      LONG(hari_main - 0x20) /* 28 : entry - 0x20 */
      LONG(0x01) /* 32 : heap領域開始アドレス */
  }

  .text : {*(.text)}

  .data 0x310000 : AT ( ADDR(.text) + SIZEOF(.text) ) {
    *(.data)
      *(.rodata*)
      *(.bss)
  }

  /DISCARD/ : { *(.eh_frame) }
}

api.ls

OUTPUT_FORMAT("binary");

SECTIONS
{
  .head 0x0 : {
    LONG(128 * 1024) /* 0x0000(DWORD): The size of data segment for API. */
    LONG(0x69726148) /* 0x0004(DWORD): Signature "Hari". */
    LONG(0) /* 0x0008(DWORD): The size of spare area in data segment. */
    LONG(0x0400) /* 0x000c(DWORD): Init value of ESP & the destination of .data. */ 
    LONG(SIZEOF(.data)) /* 0x0010(DWORD): The size of .data. */
    LONG(LOADADDR(.data)) /* 0x0014(DWORD): The file place of init value of .data. */
    LONG(0xE9000000) /* 0x0018(DWORD): 0xE9000000. JMP to the app's entry address. */
    LONG(hari_main - 0x20) /* 0x001c(DWORD): Entry address - 0x20. */
    LONG(24 * 1024) /* 0x0020(DWORD): Start address of heap area(malloc area). */
  }

  /* The area for program code. */
  /* 複数のファイルがある場合は？ 複数の .text 領域を全てここに配置する？ */
  .text : { *(.text) }

  /* The area for variables having initial values. */
  .data 0x0400 : AT ( ADDR(.text) + SIZEOF(.text) ) {
    *(.data)
    *(.rodata) /* The area for const variables. */
    *(.bss*) /* The area for variables not having initial values. */
  }

  /DISCARD/ : { *(.eh_frame) }
}

• What is an app segment by adding 0x60?
• What is the purpose of a linker script?
• How does an assembly function return values to a caller written by C?
• Why does a C compiler execute __alloca() when it allocates a variable more than 4k bytes? In the case allocating a variable less than 4k bytes, it is ok just to subtract ESP. Probably, the number of 4k bytes is affected by paging system?
• What the difference between malloc() and alloca()? In my understanding, malloc allocates memory in heap space, but alloca allocates memory in stack space.