I am trying to convert the xv6 boot code from At&t syntax to Intel syntax and I have a problem with the ljmp instruction. I am trying to learn the boot process of Intel computers and I am not particularly strong with Intel assembly.
The original AT&T syntax is ljmp $0x8, $start32.
Minimal example:
.code16
jmp 0x8:start32 # won't assemble
.code32
start32:
nop
Using as -32 -msyntax=intel -mnaked-reg foo.s with GNU Binutils 2.35.1 produces
Error: junk ':start32' after expression for the far jmp line.
I am using GNU as, and gcc tools.
There might also be other problems with the assembly such as the gdtdesc and gdt.
The full code ported to Intel syntax is:
# Start the first CPU: switch to 32-bit protectied mode, jump into C.
# The BIOS loads this code from the first sector of the hard disk into
# memory at physical address 0x7c00 and starts executing in real mode
# with cs = 0 and ip = 7c00.
.code16
.global start
start:
# Disable interrupts.
cli
# Zero data segment registers DS, ES, and SS.
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
seta20.1:
# Wait for not busy.
in al, 0x64
test al, 0x2
jnz seta20.1
# 0xd1 -> port 0x64
mov al, 0xd1
out 0x64, al
seta20.2:
# Wait for not busy.
in al, 0x64
test al, 0x2
jnz seta20.2
# 0xdf -> port 0x60
mov al, 0xdf
out 0x60, al
# Switch from real to protected mode. Use a bootstrap GDT that makes
# virtual addresses map directly to physical addressses so that the
# effective memory map doesn't change during the transition.
lgdt gdtdesc
# Protection Enable in cr0 register.
mov eax, cr0
or eax, 0x1
mov cr0, eax
# Complete the transtion to 32-bit protected mode by using a long jmp
# to reload cs and eip. The segment descriptors are set up with no
# translation, so that the mapping is still the identity mapping.
# This instruction giving me problems.
ljmp start32, 0x8
.code32
start32:
# Set up the protected-mode data segment registers
mov ax, 0x10
mov ds, ax
mov es, ax
mov ss, ax
# Zero the segments not ready for use.
xor ax, ax
mov fs, ax
mov gs, ax
# Set up the stack pointer and call into C.
mov esp, start
call bootmain
# If bootmain returns spin.. ??
spin:
hlt
jmp spin
# Bootstrap GDT set up null segment, code segment, and data segment respectively.
# Force 4 byte alignment.
.p2align 2
gdt:
.word 0x0000, 0x0000
.byte 0, 0, 0, 0
.word 0xffff, 0x0000
.byte 0, 0x9a, 0xcf, 0
.word 0xffff, 0x0000
.byte 0, 0x92, 0xcf, 0
# sizeof(gdt) - 1 and address of gdt respectively.
gdtdesc:
.word (gdtdesc - gdt - 1)
.long gdt
You can use jmp 0x08, start32
For some reason, jmp 0x8:start32 only works after .intel_syntax noprefix, even with command line args that should be equivalent. This is the syntax used by Binutils objdump -d -Mintel -mi8086, e.g. ea 16 00 08 00 jmp 0x8:0x16 so it's probably a GAS bug that it's not accepted sometimes.
I edited your question to create a small reproducible example with as 2.35.1 (which I have on Arch GNU/Linux) based on your comments replying to Jester. I included command line options: I assume you must have been using those because there's no .intel_syntax noprefix directive in your file.
That seems to be the problem: -msyntax=intel -mnaked-reg makes other Intel syntax things work, like xor ax,ax, but does not make jmp 0x8:start32 work (or other ways of writing it). Only a .intel_syntax noprefix1 directive makes that syntax for far jmp work.
# .intel_syntax noprefix # rely on command line options to set this
.code16
xor ax, ax # verify that command-line setting of intel_syntax worked, otherwise this line errors.
ljmp 0x8, start32 # Working before or after a syntax directive, but is basically AT&T syntax
# jmp 0x8:start32 # fails here, works after a directive
jmp 0x8, start32 # Michael Petch's suggested syntax that's still somewhat AT&Tish. works with just cmdline opts.
.att_syntax
ljmp $0x8, $start32 # working everywhere, even with clang
.intel_syntax noprefix
jmp 0x8:start32 # objdump disassembly syntax, but only works after a .intel_syntax noprefix directive
.code32
start32:
nop
I verified that -msyntax=intel -mnaked-reg work for other instructions where their effect is necessary: movzx ax, al works. But without -mnaked-reg we'd get "too many memory references" because "ax" and "al" would be taken as symbol names. Without or "operand size mismatch" without -msyntax=intel.
A GAS listing from as -32 -msyntax=intel -mmnemonic=intel -mnaked-reg -o foo.o foo.s -al --listing-lhs-width=2 --listing-rhs-width=140
(I'm pretty sure -mmnemonic=intel is irrelevant, and implied by syntax=intel.)
Note that you can see which instructions worked because they have machine code, and which didn't (the first jmp 0x8:start32) because the left-hand column is empty for it. The very first column would normally be addresses, but is ???? because assembly failed. (Because I uncommented the jmp 0x8:start32 to show it failing the first time, working the 2nd time.)
foo.s: Assembler messages:
foo.s:6: Error: junk `:start32' after expression
GAS LISTING foo.s page 1
1 # .intel_syntax noprefix # rely on command line options to set this
2 .code16
3 ???? 0FB6C0 movzx ax, al # verify that command-line setting of intel_syntax worked, otherwise this line errors.
4
5 ???? EA170008 00 ljmp 0x8, start32 # Working before or after a syntax directive, but is basically AT&T syntax
6 jmp 0x8:start32 # fails here, works after a directive
7 ???? EA170008 00 jmp 0x8, start32 # Michael Petch's suggested syntax that's still somewhat AT&Tish. works with just cmdline opts.
8
9 .att_syntax
10 ???? EA170008 00 ljmp $0x8, $start32 # working everywhere, even with clang
11 .intel_syntax noprefix
12 ???? EA170008 00 jmp 0x8:start32 # objdump disassembly syntax, but only works after a .intel_syntax noprefix directive
13
14 .code32
15 start32:
16 ???? 90 nop
17
(GAS does listing field widths for the left column in "words", which apparently means 32-bit chunks. That's why the 00 most-significant byte of the segment selector is separated by a space.)
Putting a label before the jmp 0x8:label didn't help; it's not an issue of forward vs. backward reference. Even jmp 0x8:23 fails to assemble.
Syntax "recommended" by disassemblers, from a working build:
objdump -drwC -Mintel -mi8086 foo.o :
foo.o: file format elf32-i386
Disassembly of section .text:
00000000 <start32-0x17>:
0: 0f b6 c0 movzx ax,al
3: ea 17 00 08 00 jmp 0x8:0x17 4: R_386_16 .text
8: ea 17 00 08 00 jmp 0x8:0x17 9: R_386_16 .text
d: ea 17 00 08 00 jmp 0x8:0x17 e: R_386_16 .text
12: ea 17 00 08 00 jmp 0x8:0x17 13: R_386_16 .text
00000017 <start32>:
17: 90 nop
llvm-objdump --mattr=+16bit-mode --x86-asm-syntax=intel -d foo.o :
00000000 <.text>:
0: 0f b6 c0 movzx ax, al
3: ea 17 00 08 00 ljmp 8, 23
8: ea 17 00 08 00 ljmp 8, 23
d: ea 17 00 08 00 ljmp 8, 23
12: ea 17 00 08 00 ljmp 8, 23
00000017 <start32>:
17: 90 nop
And BTW, I didn't get clang 11.0 to assemble any Intel-syntax versions of this with a symbol name. ljmp 8, 12 assembles with clang, but not even ljmp 8, start32. Only by switching to AT&T syntax and back could I get clang's built-in assembler (clang -m32 -masm=intel -c) to emit a 16-bit mode far jmp.
.att_syntax
ljmp $0x8, $start32 # working everywhere, even with clang
.intel_syntax noprefix
Keep in mind this direct form of far JMP is not available in 64-bit mode; perhaps that's why LLVM's built-in assembler appears to have spent less effort on it.
Footnote 1: Actually .intel_syntax prefix works, too, but never use that. Nobody want to see the franken-monster that is mov %eax, [%eax], or especially add %edx, %eax that's using dst, src order, but with AT&T decorated register names.