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.TH "YASM_ARCH" 7 "September 2004" "YASM" "YASM Architectures"
.SH NAME
yasm_arch \- YASM Architectures
.SH "SYNOPSIS"
.ad l
.hy 0
.HP 5
\fByasm\fR \fB\-a\ \fIarch\fR\fR [\fB\-m\ \fImachine\fR\fR] \fB\fI\&.\&.\&.\fR\fR
.ad
.hy

.SH "DESCRIPTION"

.PP
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\&.

.PP
The architecture and machine are selected on the \fByasm\fR(1) command line by use of the \fB\-a \fIarch\fR\fR and \fB\-m \fImachine\fR\fR command line options, respectively\&.

.SH "X86 ARCHITECTURE"

.PP
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\&.

.SS "BITS Setting"

.PP
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\&.

.PP
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 \fBBITS xx\fR assembler directive\&. The default BITS setting is determined by the object format in use\&.

.SS "BITS 64 Extensions"

.PP
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\&.

.PP
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\&.

.PP
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:

.IP
add ah, [r10]
.PP
(NASM syntax) is not a legal instruction because the use of r10 requires a REX prefix, making it impossible to use ah\&.

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

.PP
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:

.IP
mov eax, 1  ; 32\-bit instruction
.IP
mov rcx, 1  ; 64\-bit instruction
.PP
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:

.IP
push eax  ; illegal instruction
.IP
push rbx  ; 1\-byte instruction
.IP
push r11  ; 2\-byte instruction with REX prefix
.PP
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:

.IP
mov ecx, 1  ; 1 byte shorter than mov rcx, 1
.IP
and edx, 3  ; equivalent to and rdx, 3
.PP
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:

.IP
add rax, 1                  ; legal
.IP
add rax, 0xffffffff         ; sign\-extended
.IP
add rax, \-1                 ; same as above
.IP
add rax, 0xffffffffffffffff ; warning (>32 bit)
.IP
mov eax, 1                  ; 5 byte instruction
.IP
mov rax, 1                  ; 10 byte instruction
.IP
mov rbx, 0x1234567890abcdef ; 10 byte instruction
.IP
mov rcx, 0xffffffff         ; 10 byte instruction
.IP
mov ecx, \-1 ; 5 byte instruction equivalent to above
.PP
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 \fB[qword]\fR\&. Examples in NASM syntax:

.IP
mov eax, [1]    ; 32 bit, with sign extension
.IP
mov al, [rax\-1] ; 32 bit, with sign extension
.IP
mov al, [qword 0x1122334455667788] ; 64\-bit absolute
.IP
mov al, [0x1122334455667788] ; truncated to 32\-bit (warning)
.PP
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)\&.

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

.IP
mov dword [rip+10], 1
.PP
stores the value 1 ten bytes after the end of the instruction\&. \fB10\fR can also be a symbolic constant, and will be treated the same way\&. On the other hand,

.IP
mov dword [symb wrt rip], 1
.PP
stores the value 1 into the address of symbol \fBsymb\fR\&. This is distinctly different than the behavior of:

.IP
mov dword [symb+rip], 1
.PP
which takes the address of the end of the instruction, adds the address of \fBsymb\fR to it, then stores the value 1 there\&. If \fBsymb\fR is a variable, this will NOT store the value 1 into the \fBsymb\fR variable!

.SH "LC3B ARCHITECTURE"

.PP
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 \fIhttp://courses.ece.uiuc.edu/ece411/\fR for more details and example code\&. The ``lc3b'' architecture consists of only one machine: ``lc3b''\&.

.SH "SEE ALSO"

.PP
\fByasm\fR(1)

.SH "BUGS"

.PP
When using the ``x86'' architecture, it is overly easy to generate AMD64 code (using the \fBBITS 64\fR directive) and generate a 32\-bit object file (by failing to specify \fB\-m amd64\fR on the command line)\&. Similarly, specifying \fB\-m amd64\fR does not default the BITS setting to 64\&.

.SH AUTHOR
Peter Johnson <peter@tortall\&.net>.