I'm having troubles when using the uppermem parameter && vmalloc kernel parameter. Maybe somebody here can help me.
    If I add "uppermem 524288" and  set vmalloc to 256M which larger than 128M (the default) then the kernel will not boot and it will log the following on the console:

initrd extends beyond end of memory (0x200ef244>0x1ffc0000)

...MP-BISO bug : 8254 timer not connected to IO-APIC

Kernel panic - not syncing : VFS : Unable to mount root fs on unknown-block(0,0)

ps: my kernel version is 2.6.18, and my machine' main memory is 512M
Here is my grub memu.

title CentOS (2.6.18-8.e15)
root (hd1,5)

uppermen 524288
kernel /boot/vmlinuz-2.6.18-8.e15 ro root=LABEL=/ rhgb quiet vmalloc=256M
initrd /boot/initrd.img-2.6.18-8.e15.img

I add some parameters which shows in green collor.

Thanx in advance!