Tribblix: manual page: yasm

YASM_ARCH(7) Yasm Supported Architectures YASM_ARCH(7)

NAME

yasm_arch - Yasm Supported Target Architectures

SYNOPSIS

yasm -a arch [-m machine] ...

DESCRIPTION

The standard Yasm distribution includes a number of modules for
different target architectures. Each target architecture can support
one or more machine architectures.

The architecture and machine are selected on the yasm(1) command line
by use of the -a arch and -m machine command line options,
respectively.

The machine architecture may also automatically be selected by
certain object formats. For example, the "elf32" object format
selects the "x86" machine architecture by default, while the "elf64"
object format selects the "amd64" machine architecture by default.

X86 ARCHITECTURE
The "x86" architecture supports the IA-32 instruction set and
derivatives and the AMD64 instruction set. It consists of two
machines: "x86" (for the IA-32 and derivatives) and "amd64" (for the
AMD64 and derivatives). The default machine for the "x86"
architecture is the "x86" machine.

BITS Setting

The x86 architecture BITS setting specifies to Yasm the processor
mode in which the generated code is intended to execute. x86
processors can run in three different major execution modes: 16-bit,
32-bit, and on AMD64-supporting processors, 64-bit. As the x86
instruction set contains portions whose function is execution-mode
dependent (such as operand-size and address-size override prefixes),
Yasm cannot assemble x86 instructions correctly unless it is told by
the user in what processor mode the code will execute.

The BITS setting can be changed in a variety of ways. When using the
NASM-compatible parser, the BITS setting can be changed directly via
the use of the BITS xx assembler directive. The default BITS setting
is determined by the object format in use.

BITS 64 Extensions
The AMD64 architecture is a new 64-bit architecture developed by AMD,
based on the 32-bit x86 architecture. It extends the original x86
architecture by doubling the number of general purpose and SIMD
registers, extending the arithmetic operations and address space to
64 bits, as well as other features.

Recently, Intel has introduced an essentially identical version of
AMD64 called EM64T.

When an AMD64-supporting processor is executing in 64-bit mode, a
number of additional extensions are available, including extra
general purpose registers, extra SSE2 registers, and RIP-relative
addressing.

Yasm extends the base NASM syntax to support AMD64 as follows. To
enable assembly of instructions for the 64-bit mode of AMD64
processors, use the directive BITS 64. As with NASM's BITS directive,
this does not change the format of the output object file to 64 bits;
it only changes the assembler mode to assume that the instructions
being assembled will be run in 64-bit mode. To specify an AMD64
object file, use -m amd64 on the Yasm command line, or explicitly
target a 64-bit object format such as -f win64 or -f elf64. -f
elfx32 can be used to select 32-bit ELF object format for AMD64
processors.

Register Changes

The additional 64-bit general purpose registers are named r8-r15.
There are also 8-bit (rXb), 16-bit (rXw), and 32-bit (rXd)
subregisters that map to the least significant 8, 16, or 32 bits
of the 64-bit register. The original 8 general purpose registers
have also been extended to 64-bits: eax, edx, ecx, ebx, esi, edi,
esp, and ebp have new 64-bit versions called rax, rdx, rcx, rbx,
rsi, rdi, rsp, and rbp respectively. The old 32-bit registers map
to the least significant bits of the new 64-bit registers.

New 8-bit registers are also available that map to the 8 least
significant bits of rsi, rdi, rsp, and rbp. These are called sil,
dil, spl, and bpl respectively. Unfortunately, due to the way
instructions are encoded, these new 8-bit registers are encoded
the same as the old 8-bit registers ah, dh, ch, and bh. The
processor tells which is being used by the presence of the new
REX prefix that is used to specify the other extended registers.
This means it is illegal to mix the use of ah, dh, ch, and bh
with an instruction that requires the REX prefix for other
reasons. For instance:

add ah, [r10]

(NASM syntax) is not a legal instruction because the use of r10
requires a REX prefix, making it impossible to use ah.

In 64-bit mode, an additional 8 SSE2 registers are also
available. These are named xmm8-xmm15.

64 Bit Instructions

By default, most operations in 64-bit mode remain 32-bit;
operations that are 64-bit usually require a REX prefix (one bit
in the REX prefix determines whether an operation is 64-bit or
32-bit). Thus, essentially all 32-bit instructions have a 64-bit
version, and the 64-bit versions of instructions can use extended
registers "for free" (as the REX prefix is already present).
Examples in NASM syntax:

mov eax, 1 ; 32-bit instruction

mov rcx, 1 ; 64-bit instruction

Instructions that modify the stack (push, pop, call, ret, enter,
and leave) are implicitly 64-bit. Their 32-bit counterparts are
not available, but their 16-bit counterparts are. Examples in
NASM syntax:

push eax ; illegal instruction

push rbx ; 1-byte instruction

push r11 ; 2-byte instruction with REX prefix

Implicit Zero Extension

Results of 32-bit operations are implicitly zero-extended to the
upper 32 bits of the corresponding 64-bit register. 16 and 8 bit
operations, on the other hand, do not affect upper bits of the
register (just as in 32-bit and 16-bit modes). This can be used
to generate smaller code in some instances. Examples in NASM
syntax:

mov ecx, 1 ; 1 byte shorter than mov rcx, 1

and edx, 3 ; equivalent to and rdx, 3

Immediates

For most instructions in 64-bit mode, immediate values remain 32
bits; their value is sign-extended into the upper 32 bits of the
target register prior to being used. The exception is the mov
instruction, which can take a 64-bit immediate when the
destination is a 64-bit register. Examples in NASM syntax:

add rax, 1 ; optimized down to signed 8-bit

add rax, dword 1 ; force size to 32-bit

add rax, 0xffffffff ; sign-extended 32-bit

add rax, -1 ; same as above

add rax, 0xffffffffffffffff ; truncated to 32-bit (warning)

mov eax, 1 ; 5 byte

mov rax, 1 ; 5 byte (optimized to signed 32-bit)

mov rax, qword 1 ; 10 byte (forced 64-bit)

mov rbx, 0x1234567890abcdef ; 10 byte

mov rcx, 0xffffffff ; 10 byte (does not fit in signed 32-bit)

mov ecx, -1 ; 5 byte, equivalent to above

mov rcx, sym ; 5 byte, 32-bit size default for symbols

mov rcx, qword sym ; 10 byte, override default size

The handling of mov reg64, unsized immediate is different between
YASM and NASM 2.x; YASM follows the above behavior, while NASM
2.x does the following:

add rax, 0xffffffff ; sign-extended 32-bit immediate

add rax, -1 ; same as above

add rax, 0xffffffffffffffff ; truncated 32-bit (warning)

add rax, sym ; sign-extended 32-bit immediate

mov eax, 1 ; 5 byte (32-bit immediate)

mov rax, 1 ; 10 byte (64-bit immediate)

mov rbx, 0x1234567890abcdef ; 10 byte instruction

mov rcx, 0xffffffff ; 10 byte instruction

mov ecx, -1 ; 5 byte, equivalent to above

mov ecx, sym ; 5 byte (32-bit immediate)

mov rcx, sym ; 10 byte instruction

mov rcx, qword sym ; 10 byte (64-bit immediate)

Displacements

Just like immediates, displacements, for the most part, remain 32
bits and are sign extended prior to use. Again, the exception is
one restricted form of the mov instruction: between the
al/ax/eax/rax register and a 64-bit absolute address (no
registers allowed in the effective address). In NASM syntax, use
of the 64-bit absolute form requires [qword]. Examples in NASM
syntax:

mov eax, [1] ; 32 bit, with sign extension

mov al, [rax-1] ; 32 bit, with sign extension

mov al, [qword 0x1122334455667788] ; 64-bit absolute

mov al, [0x1122334455667788] ; truncated to 32-bit (warning)

RIP Relative Addressing

In 64-bit mode, a new form of effective addressing is available
to make it easier to write position-independent code. Any memory
reference may be made RIP relative (RIP is the instruction
pointer register, which contains the address of the location
immediately following the current instruction).

In NASM syntax, there are two ways to specify RIP-relative
addressing:

mov dword [rip+10], 1

stores the value 1 ten bytes after the end of the instruction.
10 can also be a symbolic constant, and will be treated the same
way. On the other hand,

mov dword [symb wrt rip], 1

stores the value 1 into the address of symbol symb. This is
distinctly different than the behavior of:

mov dword [symb+rip], 1

which takes the address of the end of the instruction, adds the
address of symb to it, then stores the value 1 there. If symb is
a variable, this will not store the value 1 into the symb
variable!

Yasm also supports the following syntax for RIP-relative
addressing:

mov [rel sym], rax ; RIP-relative

mov [abs sym], rax ; not RIP-relative

The behavior of:

mov [sym], rax

Depends on a mode set by the DEFAULT directive, as follows. The
default mode is always "abs", and in "rel" mode, use of
registers, an fs or gs segment override, or an explicit "abs"
override will result in a non-RIP-relative effective address.

default rel

mov [sym], rbx ; RIP-relative

mov [abs sym], rbx ; not RIP-relative (explicit override)

mov [rbx+1], rbx ; not RIP-relative (register use)

mov [fs:sym], rbx ; not RIP-relative (fs or gs use)

mov [ds:sym], rbx ; RIP-relative (segment, but not fs or gs)

mov [rel sym], rbx ; RIP-relative (redundant override)

default abs

mov [sym], rbx ; not RIP-relative

mov [abs sym], rbx ; not RIP-relative

mov [rbx+1], rbx ; not RIP-relative

mov [fs:sym], rbx ; not RIP-relative

mov [ds:sym], rbx ; not RIP-relative

mov [rel sym], rbx ; RIP-relative (explicit override)

Memory references

Usually the size of a memory reference can be deduced by which
registers you're moving--for example, "mov [rax],ecx" is a 32-bit
move, because ecx is 32 bits. YASM currently gives the
non-obvious "invalid combination of opcode and operands" error if
it can't figure out how much memory you're moving. The fix in
this case is to add a memory size specifier: qword, dword, word,
or byte.

Here's a 64-bit memory move, which sets 8 bytes starting at rax:

mov qword [rax], 1

Here's a 32-bit memory move, which sets 4 bytes:

mov dword [rax], 1

Here's a 16-bit memory move, which sets 2 bytes:

mov word [rax], 1

Here's an 8-bit memory move, which sets 1 byte:

mov byte [rax], 1

LC3B ARCHITECTURE
The "lc3b" architecture supports the LC-3b ISA as used in the ECE 312
(now ECE 411) course at the University of Illinois, Urbana-Champaign,
as well as other university courses. See
http://courses.ece.uiuc.edu/ece411/ for more details and example
code. The "lc3b" architecture consists of only one machine: "lc3b".

BUGS

When using the "x86" architecture, it is overly easy to generate
AMD64 code (using the BITS 64 directive) and generate a 32-bit object
file (by failing to specify -m amd64 on the command line or selecting
a 64-bit object format). Similarly, specifying -m amd64 does not
default the BITS setting to 64. An easy way to avoid this is by
directly specifying a 64-bit object format such as -f elf64.

AUTHOR

Peter Johnson <peter@tortall.net>
Author.