Significance of link address 2000 8000 7C00;

Question

Can you explain to me the significance of link address 2000 8000 7C00? It's in a MACRO from the GRUB configure script, which checks whether OBJCOPY works for absolute addresses.

Here's the snippet of the code:

AC_DEFUN([grub_PROG_OBJCOPY_ABSOLUTE],
[AC_MSG_CHECKING([whether ${OBJCOPY} works for absolute addresses])
AC_CACHE_VAL(grub_cv_prog_objcopy_absolute,
[cat > conftest.c <<\EOF
void
cmain (void)
{
   *((int *) 0x1000) = 2;
}
EOF

if AC_TRY_EVAL(ac_compile) && test -s conftest.o; then :
else
  AC_MSG_ERROR([${CC-cc} cannot compile C source code])
fi
grub_cv_prog_objcopy_absolute=yes
for link_addr in 2000 8000 7C00; do
  if AC_TRY_COMMAND([${CC-cc} ${CFLAGS} -nostdlib -Wl,-N -Wl,-Ttext -Wl,$link_addr conftest.o -o conftest.exec]); then :
  else
    AC_MSG_ERROR([${CC-cc} cannot link at address $link_addr])
  fi
  if AC_TRY_COMMAND([${OBJCOPY-objcopy} -O binary conftest.exec conftest]); then :
  else
    AC_MSG_ERROR([${OBJCOPY-objcopy} cannot create binary files])
  fi
  if test ! -f conftest.old || AC_TRY_COMMAND([cmp -s conftest.old conftest]); then
    mv -f conftest conftest.old
  else
    grub_cv_prog_objcopy_absolute=no
    break
  fi
done

I can't help you with 2000 and 8000, but 0x7C00 is the address where the BIOS loads the Master Boot Record to. Google "7C00 mbr" will produce more reading and better explanations. I couldn't find anything for 0x2000 and 0x8000 right away. — Johann Aydinbas, Feb 01 '19 at 01:55

score 3 · Accepted Answer · answered Feb 01 '19 at 08:06

Taken from gnu.org - Hacking GRUB

GRUB consists of two distinct components, called stages, which are loaded at different times in the boot process. Because they run mutual-exclusively, sometimes a memory area overlaps with another memory area. And, even in one stage, a single memory area can be used for various purposes, because their usages are mutually exclusive.

0x7C00 Stage 1 is loaded here by BIOS or another boot loader

0x2000 The optional Stage 1.5 is loaded here / Command-line buffer for Multiboot kernels and modules

0x8000 Stage2 is loaded here

Significance of link address 2000 8000 7C00;

1 Answers1