I was wondering what highmem parameter affect to the system, so I tried to dig into the kernel code. But, I had to spend so many hours to find the starting point.

This is the code, I finally found and I want to keep it here for later use.

http://lxr.linux.no/linux+v2.6.11/arch/i386/kernel/setup.c

static void __init parse_cmdline_early (char ** cmdline_p)
{
        char c = ' ', *to = command_line, *from = saved_command_line;
        int len = 0;
        int userdef = 0;

        /* Save unparsed command line copy for /proc/cmdline */
        saved_command_line[COMMAND_LINE_SIZE-1] = '';

        for (;;) {
                if (c != ' ')
                        goto next_char;
                /*
                 * "mem=nopentium" disables the 4MB page tables.
                 * "mem=XXX[kKmM]" defines a memory region from HIGH_MEM
                 * to <mem>, overriding the bios size.
                 * "memmap=XXX[KkmM]@XXX[KkmM]" defines a memory region from
                 * <start> to <start>+<mem>, overriding the bios size.
                 *
                 * HPA tells me bootloaders need to parse mem=, so no new
                 * option should be mem=  [also see Documentation/i386/boot.txt]
                 */
                if (!memcmp(from, "mem=", 4)) {
                        if (to != command_line)
                                to--;
                        if (!memcmp(from+4, "nopentium", 9)) {
                                from += 9+4;
                                clear_bit(X86_FEATURE_PSE, boot_cpu_data.x86_capability);
                                disable_pse = 1;
                        } else {
                                /* If the user specifies memory size, we
                                 * limit the BIOS-provided memory map to
                                 * that size. exactmap can be used to specify
                                 * the exact map. mem=number can be used to
                                 * trim the existing memory map.
                                 */
                                unsigned long long mem_size;

                                mem_size = memparse(from+4, &from);
                                limit_regions(mem_size);
                                userdef=1;
                        }
                }

                else if (!memcmp(from, "memmap=", 7)) {
                        if (to != command_line)
                                to--;
                        if (!memcmp(from+7, "exactmap", 8)) {
                                from += 8+7;
                                e820.nr_map = 0;
                                userdef = 1;
                        } else {
                                /* If the user specifies memory size, we
                                 * limit the BIOS-provided memory map to
                                 * that size. exactmap can be used to specify
                                 * the exact map. mem=number can be used to
                                 * trim the existing memory map.
                                 */
                                unsigned long long start_at, mem_size;

                                mem_size = memparse(from+7, &from);
                                if (*from == '@') {
                                        start_at = memparse(from+1, &from);
                                        add_memory_region(start_at, mem_size, E820_RAM);
                                } else if (*from == '#') {
                                        start_at = memparse(from+1, &from);
                                        add_memory_region(start_at, mem_size, E820_ACPI);
                                } else if (*from == '$') {
                                        start_at = memparse(from+1, &from);
                                        add_memory_region(start_at, mem_size, E820_RESERVED);
                                } else {
                                        limit_regions(mem_size);
                                        userdef=1;
                                }
                        }
                }

                else if (!memcmp(from, "noexec=", 7))
                        noexec_setup(from + 7);


#ifdef  CONFIG_X86_SMP
                /*
                 * If the BIOS enumerates physical processors before logical,
                 * maxcpus=N at enumeration-time can be used to disable HT.
                 */
                else if (!memcmp(from, "maxcpus=", 8)) {
                        extern unsigned int maxcpus;

                        maxcpus = simple_strtoul(from + 8, NULL, 0);
                }
#endif

#ifdef CONFIG_ACPI_BOOT
                /* "acpi=off" disables both ACPI table parsing and interpreter */
                else if (!memcmp(from, "acpi=off", 8)) {
                        disable_acpi();
                }

                /* acpi=force to over-ride black-list */
                else if (!memcmp(from, "acpi=force", 10)) {
                        acpi_force = 1;
                        acpi_ht = 1;
                        acpi_disabled = 0;
                }

                /* acpi=strict disables out-of-spec workarounds */
                else if (!memcmp(from, "acpi=strict", 11)) {
                        acpi_strict = 1;
                }

                /* Limit ACPI just to boot-time to enable HT */
                else if (!memcmp(from, "acpi=ht", 7)) {
                        if (!acpi_force)
                                disable_acpi();
                        acpi_ht = 1;
                }

                /* "pci=noacpi" disable ACPI IRQ routing and PCI scan */
                else if (!memcmp(from, "pci=noacpi", 10)) {
                        acpi_disable_pci();
                }
                /* "acpi=noirq" disables ACPI interrupt routing */
                else if (!memcmp(from, "acpi=noirq", 10)) {
                        acpi_noirq_set();
                }

                else if (!memcmp(from, "acpi_sci=edge", 13))
                        acpi_sci_flags.trigger =  1;

                else if (!memcmp(from, "acpi_sci=level", 14))
                        acpi_sci_flags.trigger = 3;

                else if (!memcmp(from, "acpi_sci=high", 13))
                        acpi_sci_flags.polarity = 1;

                else if (!memcmp(from, "acpi_sci=low", 12))
                        acpi_sci_flags.polarity = 3;

#ifdef CONFIG_X86_IO_APIC
                else if (!memcmp(from, "acpi_skip_timer_override", 24))
                        acpi_skip_timer_override = 1;
#endif

#ifdef CONFIG_X86_LOCAL_APIC
                /* disable IO-APIC */
                else if (!memcmp(from, "noapic", 6))
                        disable_ioapic_setup();
#endif /* CONFIG_X86_LOCAL_APIC */
#endif /* CONFIG_ACPI_BOOT */

                /*
                 * highmem=size forces highmem to be exactly 'size' bytes.
                 * This works even on boxes that have no highmem otherwise.
                 * This also works to reduce highmem size on bigger boxes.
                 */
                else if (!memcmp(from, "highmem=", 8))
                        highmem_pages = memparse(from+8, &from) >> PAGE_SHIFT;

                /*
                 * vmalloc=size forces the vmalloc area to be exactly 'size'
                 * bytes. This can be used to increase (or decrease) the
                 * vmalloc area - the default is 128m.
                 */
                else if (!memcmp(from, "vmalloc=", 8))
                        __VMALLOC_RESERVE = memparse(from+8, &from);

        next_char:
                c = *(from++);
                if (!c)
                        break;
                if (COMMAND_LINE_SIZE <= ++len)
                        break;
                *(to++) = c;
        }
        *to = '';
        *cmdline_p = command_line;
        if (userdef) {
                printk(KERN_INFO "user-defined physical RAM map:n");
                print_memory_map("user");
        }
}