/* * linux_trampoline * * Copyright (C) 2013 Patrick Georgi <patrick@georgi-clan.de> * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; version 2 of the License. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. */ /* NOTE: THIS CODE MUST REMAIN POSITION INDEPENDENT * IT SHOULDN'T USE THE STACK * AND IN GENERAL EXPECT NOTHING BUT RAM TO WORK */ .code32 .data #include "linux_trampoline.h" #define HEADER_SIG 0x4f49424c // LBIO little endian #define CB_TAG_FORWARD 0x11 #define CB_TAG_MEMORY 0x1 #define CB_TAG_FRAMEBUFFER 0x12 #define E820_NR_OFFSET 0x1e8 #define LINUX_ENTRY_OFFSET 0x214 #define E820_OFFSET 0x2d0 .trampoline_start: cld xor %edx, %edx mov $0, %ecx .headerSearch: mov $0x10000, %ebx add %ecx, %ebx mov (%ecx), %eax cmp $HEADER_SIG, %eax je .headerSearchDone // found the header add $16, %ecx cmp %ecx, %ebx jne .headerSearch .headerSearchDone: cmp %ecx, %ebx // reached the end == not found anything? je 2f // give up // we assume the checksum is okay, no test mov 4(%ecx), %ebx add %ecx, %ebx // ebx = cb_header + header_bytes mov 20(%ecx), %ecx // ecx = table_entries .tableScan: cmp $CB_TAG_FORWARD, (%ebx) jne .testMemory /* forward tag: assume 32bit pointer */ mov 8(%ebx), %ecx jmp .headerSearch .testMemory: cmp $CB_TAG_MEMORY, (%ebx) jne .testFramebuffer /* memory tag: copy e820 map and entry count. also determine alt_mem_k */ mov 4(%ebx), %eax sub $8, %eax shr $2, %eax /* eax = number of dwords of e820 data */ cmp $(32 * 5), %eax /* linux wants at most 32 entries of 5 dwords */ jng 1f mov $(32 * 5), %eax /* only copy 32 entries */ 1: mov %eax, %esi mov $5, %edi div %edi mov %eax, (LINUX_PARAM_LOC + E820_NR_OFFSET) mov %esi, %eax xchg %eax, %ecx lea 8(%ebx), %esi /* e820 data source */ mov $(LINUX_PARAM_LOC + E820_OFFSET), %edi rep movsl xchg %eax, %ecx jmp .endScan .testFramebuffer: cmp $CB_TAG_FRAMEBUFFER, (%ebx) jne .endScan /* TODO: handle framebuffer tag */ .endScan: add 4(%ebx), %ebx dec %ecx jnz .tableScan /* Setup basic code and data segment selectors for Linux ** ** Flat code segment descriptor: ** selector: 0x10 ** base : 0x00000000 ** limit : 0xFFFFFFFF ** type : code, execute, read ** ** Flat data segment descriptor: ** selector: 0x18 ** base : 0x00000000 ** limit : 0xFFFFFFFF ** type : data, read/write ** ** Use TRAMPOLINE_ENTRY_LOC as a scratchpad. */ mov $TRAMPOLINE_ENTRY_LOC, %eax movl $0x0000ffff, 16(%eax) // Set up the 2 new descriptors movl $0x00cf9b00, 20(%eax) movl $0x0000ffff, 24(%eax) movl $0x00cf9300, 28(%eax) movb $0x2b, 0(%eax) // Set the size movl %eax, 2(%eax) // Set pointer to new GDT lgdt (%eax) // Load it /* finally: jump to kernel */ mov $LINUX_PARAM_LOC, %esi jmp *(LINUX_PARAM_LOC + LINUX_ENTRY_OFFSET) 2: hlt jmp 2b .trampoline_end: