SYNOPSIS
yasm -a arch [-m machine] ...
DESCRIPTION
The standard YASM distribution includes a number of loadable modules
for different target architectures. Additional target architectures may
be installed as third-party modules. 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.
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
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.
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
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 ; legal
add rax, 0xffffffff ; sign-extended
add rax, -1 ; same as above
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!
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”.
SEE ALSO
YASM September 2004 YASM_ARCH(7)
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