binutils-gdb/gas/doc/c-s390.texi

@c Copyright 2009, 2011
@c Free Software Foundation, Inc.
@c This is part of the GAS manual.
@c For copying conditions, see the file as.texinfo.
@ifset GENERIC
@page
@node S/390-Dependent
@chapter IBM S/390 Dependent Features
@end ifset
@ifclear GENERIC
@node Machine Dependencies
@chapter IBM S/390 Dependent Features
@end ifclear

@cindex s390 support

The s390 version of @code{@value{AS}} supports two architectures modes
and seven chip levels. The architecture modes are the Enterprise System
Architecture (ESA) and the newer z/Architecture mode. The chip levels
are g5, g6, z900, z990, z9-109, z9-ec, z10, z196, and zEC12.

@menu
* s390 Options::                Command-line Options.
* s390 Characters::		Special Characters.
* s390 Syntax::                 Assembler Instruction syntax.
* s390 Directives::             Assembler Directives.
* s390 Floating Point::         Floating Point.
@end menu

@node s390 Options
@section Options
@cindex options for s390
@cindex s390 options

The following table lists all available s390 specific options:

@table @code
@cindex @samp{-m31} option, s390
@cindex @samp{-m64} option, s390
@item -m31 | -m64
Select 31- or 64-bit ABI implying a word size of 32- or 64-bit.

These options are only available with the ELF object file format, and
require that the necessary BFD support has been included (on a 31-bit
platform you must add --enable-64-bit-bfd on the call to the configure
script to enable 64-bit usage and use s390x as target platform).

@cindex @samp{-mesa} option, s390
@cindex @samp{-mzarch} option, s390
@item -mesa | -mzarch
Select the architecture mode, either the Enterprise System Architecture
(esa) mode or the z/Architecture mode (zarch).

The 64-bit instructions are only available with the z/Architecture mode.
The combination of @samp{-m64} and @samp{-mesa} results in a warning
message.

@cindex @samp{-march=} option, s390
@item -march=@var{CPU}
This option specifies the target processor. The following processor names
are recognized:
@code{g5},
@code{g6},
@code{z900},
@code{z990},
@code{z9-109},
@code{z9-ec},
@code{z10} and
@code{z196}.
Assembling an instruction that is not supported on the target processor
results in an error message. Do not specify @code{g5} or @code{g6}
with @samp{-mzarch}.

@cindex @samp{-mregnames} option, s390
@item -mregnames
Allow symbolic names for registers.

@cindex @samp{-mno-regnames} option, s390
@item -mno-regnames
Do not allow symbolic names for registers.

@cindex @samp{-mwarn-areg-zero} option, s390
@item -mwarn-areg-zero
Warn whenever the operand for a base or index register has been specified
but evaluates to zero. This can indicate the misuse of general purpose
register 0 as an address register.

@end table

@node s390 Characters
@section Special Characters
@cindex line comment character, s390
@cindex s390 line comment character

@samp{#} is the line comment character.

If a @samp{#} appears as the first character of a line then the whole
line is treated as a comment, but in this case the line could also be
a logical line number directive (@pxref{Comments}) or a preprocessor
control command (@pxref{Preprocessing}).

@cindex line separator, s390
@cindex statement separator, s390
@cindex s390 line separator
The @samp{;} character can be used instead of a newline to separate
statements.

@node s390 Syntax
@section Instruction syntax
@cindex instruction syntax, s390
@cindex s390 instruction syntax

The assembler syntax closely follows the syntax outlined in
Enterprise Systems Architecture/390 Principles of Operation (SA22-7201)
and the z/Architecture Principles of Operation (SA22-7832).

Each instruction has two major parts, the instruction mnemonic
and the instruction operands. The instruction format varies.

@menu
* s390 Register::               Register Naming
* s390 Mnemonics::              Instruction Mnemonics
* s390 Operands::               Instruction Operands
* s390 Formats::                Instruction Formats
* s390 Aliases::		Instruction Aliases
* s390 Operand Modifier::       Instruction Operand Modifier
* s390 Instruction Marker::     Instruction Marker
* s390 Literal Pool Entries::   Literal Pool Entries
@end menu

@node s390 Register
@subsection Register naming
@cindex register naming, s390
@cindex s390 register naming

The @code{@value{AS}} recognizes a number of predefined symbols for the
various processor registers. A register specification in one of the
instruction formats is an unsigned integer between 0 and 15. The specific
instruction and the position of the register in the instruction format
denotes the type of the register. The register symbols are prefixed with
@samp{%}:

@display
@multitable {%rN} {the 16 general purpose registers, 0 <= N <= 15}
@item %rN @tab the 16 general purpose registers, 0 <= N <= 15
@item %fN @tab the 16 floating point registers, 0 <= N <= 15
@item %aN @tab the 16 access registers, 0 <= N <= 15
@item %cN @tab the 16 control registers, 0 <= N <= 15
@item %lit @tab an alias for the general purpose register %r13
@item %sp @tab an alias for the general purpose register %r15
@end multitable
@end display

@node s390 Mnemonics
@subsection Instruction Mnemonics
@cindex instruction mnemonics, s390
@cindex s390 instruction mnemonics

All instructions documented in the Principles of Operation are supported
with the mnemonic and order of operands as described.
The instruction mnemonic identifies the instruction format
(@ref{s390 Formats}) and the specific operation code for the instruction.
For example, the @samp{lr} mnemonic denotes the instruction format @samp{RR}
with the operation code @samp{0x18}.

The definition of the various mnemonics follows a scheme, where the first
character usually hint at the type of the instruction:

@display
@multitable {sla, sll} {if r is the last character the instruction operates on registers}
@item a @tab add instruction, for example @samp{al} for add logical 32-bit
@item b @tab branch instruction, for example @samp{bc} for branch on condition
@item c @tab compare or convert instruction, for example @samp{cr} for compare
register 32-bit
@item d @tab divide instruction, for example @samp{dlr} devide logical register
64-bit to 32-bit
@item i @tab insert instruction, for example @samp{ic} insert character
@item l @tab load instruction, for example @samp{ltr} load and test register
@item mv @tab move instruction, for example @samp{mvc} move character
@item m @tab multiply instruction, for example @samp{mh} multiply halfword
@item n @tab and instruction, for example @samp{ni} and immediate
@item o @tab or instruction, for example @samp{oc} or character
@item sla, sll @tab shift left single instruction
@item sra, srl @tab shift right single instruction
@item st @tab store instruction, for example @samp{stm} store multiple
@item s @tab subtract instruction, for example @samp{slr} subtract
logical 32-bit
@item t @tab test or translate instruction, of example @samp{tm} test under mask
@item x @tab exclusive or instruction, for example @samp{xc} exclusive or
character
@end multitable
@end display

Certain characters at the end of the mnemonic may describe a property
of the instruction:

@display
@multitable {c} {if r is the last character the instruction operates on registers}
@item c @tab the instruction uses a 8-bit character operand
@item f @tab the instruction extends a 32-bit operand to 64 bit
@item g @tab the operands are treated as 64-bit values
@item h @tab the operand uses a 16-bit halfword operand
@item i @tab the instruction uses an immediate operand
@item l @tab the instruction uses unsigned, logical operands
@item m @tab the instruction uses a mask or operates on multiple values
@item r @tab if r is the last character, the instruction operates on registers
@item y @tab the instruction uses 20-bit displacements
@end multitable
@end display

There are many exceptions to the scheme outlined in the above lists, in
particular for the priviledged instructions. For non-priviledged
instruction it works quite well, for example the instruction @samp{clgfr}
c: compare instruction, l: unsigned operands, g: 64-bit operands,
f: 32- to 64-bit extension, r: register operands. The instruction compares
an 64-bit value in a register with the zero extended 32-bit value from
a second register.
For a complete list of all mnemonics see appendix B in the Principles
of Operation.

@node s390 Operands
@subsection Instruction Operands
@cindex instruction operands, s390
@cindex s390 instruction operands

Instruction operands can be grouped into three classes, operands located
in registers, immediate operands, and operands in storage.

A register operand can be located in general, floating-point, access,
or control register. The register is identified by a four-bit field.
The field containing the register operand is called the R field.

Immediate operands are contained within the instruction and can have
8, 16 or 32 bits. The field containing the immediate operand is called
the I field. Dependent on the instruction the I field is either signed
or unsigned.

A storage operand consists of an address and a length. The address of a
storage operands can be specified in any of these ways:

@itemize
@item The content of a single general R
@item The sum of the content of a general register called the base
register B plus the content of a displacement field D
@item The sum of the contents of two general registers called the
index register X and the base register B plus the content of a
displacement field
@item The sum of the current instruction address and a 32-bit signed
immediate field multiplied by two.
@end itemize

The length of a storage operand can be:

@itemize
@item Implied by the instruction
@item Specified by a bitmask
@item Specified by a four-bit or eight-bit length field L
@item Specified by the content of a general register
@end itemize

The notation for storage operand addresses formed from multiple fields is
as follows:

@table @code
@item Dn(Bn)
the address for operand number n is formed from the content of general
register Bn called the base register and the displacement field Dn.
@item Dn(Xn,Bn)
the address for operand number n is formed from the content of general
register Xn called the index register, general register Bn called the
base register and the displacement field Dn.
@item Dn(Ln,Bn)
the address for operand number n is formed from the content of general
regiser Bn called the base register and the displacement field Dn.
The length of the operand n is specified by the field Ln.
@end table

The base registers Bn and the index registers Xn of a storage operand can
be skipped. If Bn and Xn are skipped, a zero will be stored to the operand
field. The notation changes as follows:

@display
@multitable @columnfractions 0.30 0.30
@headitem full notation @tab short notation
@item Dn(0,Bn) @tab Dn(Bn)
@item Dn(0,0) @tab Dn
@item Dn(0) @tab Dn
@item Dn(Ln,0) @tab Dn(Ln)
@end multitable
@end display


@node s390 Formats
@subsection Instruction Formats
@cindex instruction formats, s390
@cindex s390 instruction formats

The Principles of Operation manuals lists 26 instruction formats where
some of the formats have multiple variants. For the @samp{.insn}
pseudo directive the assembler recognizes some of the formats.
Typically, the most general variant of the instruction format is used
by the @samp{.insn} directive.

The following table lists the abbreviations used in the table of
instruction formats:

@display
@multitable {OpCode / OpCd} {Displacement lower 12 bits for operand x.}
@item OpCode / OpCd @tab Part of the op code.
@item Bx @tab Base register number for operand x.
@item Dx @tab Displacement for operand x.
@item DLx @tab Displacement lower 12 bits for operand x.
@item DHx @tab Displacement higher 8-bits for operand x.
@item Rx @tab Register number for operand x.
@item Xx @tab Index register number for operand x.
@item Ix @tab Signed immediate for operand x.
@item Ux @tab Unsigned immediate for operand x.
@end multitable
@end display

An instruction is two, four, or six bytes in length and must be aligned
on a 2 byte boundary. The first two bits of the instruction specify the
length of the instruction, 00 indicates a two byte instruction, 01 and 10
indicates a four byte instruction, and 11 indicates a six byte instruction.

The following table lists the s390 instruction formats that are available
with the @samp{.insn} pseudo directive:

@table @code
@item E format
@verbatim
+-------------+
|    OpCode   |
+-------------+
0            15
@end verbatim

@item RI format: <insn> R1,I2
@verbatim
+--------+----+----+------------------+
| OpCode | R1 |OpCd|        I2        |
+--------+----+----+------------------+
0        8    12   16                31
@end verbatim

@item RIE format: <insn> R1,R3,I2
@verbatim
+--------+----+----+------------------+--------+--------+
| OpCode | R1 | R3 |        I2        |////////| OpCode |
+--------+----+----+------------------+--------+--------+
0        8    12   16                 32       40      47
@end verbatim

@item RIL format: <insn> R1,I2
@verbatim
+--------+----+----+------------------------------------+
| OpCode | R1 |OpCd|                  I2                |
+--------+----+----+------------------------------------+
0        8    12   16                                  47
@end verbatim

@item RILU format: <insn> R1,U2
@verbatim
+--------+----+----+------------------------------------+
| OpCode | R1 |OpCd|                  U2                |
+--------+----+----+------------------------------------+
0        8    12   16                                  47
@end verbatim

@item RIS format: <insn> R1,I2,M3,D4(B4)
@verbatim
+--------+----+----+----+-------------+--------+--------+
| OpCode | R1 | M3 | B4 |     D4      |   I2   | Opcode |
+--------+----+----+----+-------------+--------+--------+
0        8    12   16   20            32       36      47
@end verbatim

@item RR format: <insn> R1,R2
@verbatim
+--------+----+----+
| OpCode | R1 | R2 |
+--------+----+----+
0        8    12  15
@end verbatim

@item RRE format: <insn> R1,R2
@verbatim
+------------------+--------+----+----+
|      OpCode      |////////| R1 | R2 |
+------------------+--------+----+----+
0                  16       24   28  31
@end verbatim

@item RRF format: <insn> R1,R2,R3,M4
@verbatim
+------------------+----+----+----+----+
|      OpCode      | R3 | M4 | R1 | R2 |
+------------------+----+----+----+----+
0                  16   20   24   28  31
@end verbatim

@item RRS format: <insn> R1,R2,M3,D4(B4)
@verbatim
+--------+----+----+----+-------------+----+----+--------+
| OpCode | R1 | R3 | B4 |     D4      | M3 |////| OpCode |
+--------+----+----+----+-------------+----+----+--------+
0        8    12   16   20            32   36   40      47
@end verbatim

@item RS format: <insn> R1,R3,D2(B2)
@verbatim
+--------+----+----+----+-------------+
| OpCode | R1 | R3 | B2 |     D2      |
+--------+----+----+----+-------------+
0        8    12   16   20           31
@end verbatim

@item RSE format: <insn> R1,R3,D2(B2)
@verbatim
+--------+----+----+----+-------------+--------+--------+
| OpCode | R1 | R3 | B2 |     D2      |////////| OpCode |
+--------+----+----+----+-------------+--------+--------+
0        8    12   16   20            32       40      47
@end verbatim

@item RSI format: <insn> R1,R3,I2
@verbatim
+--------+----+----+------------------------------------+
| OpCode | R1 | R3 |                  I2                |
+--------+----+----+------------------------------------+
0        8    12   16                                  47
@end verbatim

@item RSY format: <insn> R1,R3,D2(B2)
@verbatim
+--------+----+----+----+-------------+--------+--------+
| OpCode | R1 | R3 | B2 |    DL2      |  DH2   | OpCode |
+--------+----+----+----+-------------+--------+--------+
0        8    12   16   20            32       40      47
@end verbatim

@item RX format: <insn> R1,D2(X2,B2)
@verbatim
+--------+----+----+----+-------------+
| OpCode | R1 | X2 | B2 |     D2      |
+--------+----+----+----+-------------+
0        8    12   16   20           31
@end verbatim

@item RXE format: <insn> R1,D2(X2,B2)
@verbatim
+--------+----+----+----+-------------+--------+--------+
| OpCode | R1 | X2 | B2 |     D2      |////////| OpCode |
+--------+----+----+----+-------------+--------+--------+
0        8    12   16   20            32       40      47
@end verbatim

@item RXF format: <insn> R1,R3,D2(X2,B2)
@verbatim
+--------+----+----+----+-------------+----+---+--------+
| OpCode | R3 | X2 | B2 |     D2      | R1 |///| OpCode |
+--------+----+----+----+-------------+----+---+--------+
0        8    12   16   20            32   36  40      47
@end verbatim

@item RXY format: <insn> R1,D2(X2,B2)
@verbatim
+--------+----+----+----+-------------+--------+--------+
| OpCode | R1 | X2 | B2 |     DL2     |   DH2  | OpCode |
+--------+----+----+----+-------------+--------+--------+
0        8    12   16   20            32   36   40      47
@end verbatim

@item S format: <insn> D2(B2)
@verbatim
+------------------+----+-------------+
|      OpCode      | B2 |     D2      |
+------------------+----+-------------+
0                  16   20           31
@end verbatim

@item SI format: <insn> D1(B1),I2
@verbatim
+--------+---------+----+-------------+
| OpCode |   I2    | B1 |     D1      |
+--------+---------+----+-------------+
0        8         16   20           31
@end verbatim

@item SIY format: <insn> D1(B1),U2
@verbatim
+--------+---------+----+-------------+--------+--------+
| OpCode |   I2    | B1 |     DL1     |  DH1   | OpCode |
+--------+---------+----+-------------+--------+--------+
0        8         16   20            32   36   40      47
@end verbatim

@item SIL format: <insn> D1(B1),I2
@verbatim
+------------------+----+-------------+-----------------+
|      OpCode      | B1 |      D1     |       I2        |
+------------------+----+-------------+-----------------+
0                  16   20            32               47
@end verbatim

@item SS format: <insn> D1(R1,B1),D2(B3),R3
@verbatim
+--------+----+----+----+-------------+----+------------+
| OpCode | R1 | R3 | B1 |     D1      | B2 |     D2     |
+--------+----+----+----+-------------+----+------------+
0        8    12   16   20            32   36          47
@end verbatim

@item SSE format: <insn> D1(B1),D2(B2)
@verbatim
+------------------+----+-------------+----+------------+
|      OpCode      | B1 |     D1      | B2 |     D2     |
+------------------+----+-------------+----+------------+
0        8    12   16   20            32   36           47
@end verbatim

@item SSF format: <insn> D1(B1),D2(B2),R3
@verbatim
+--------+----+----+----+-------------+----+------------+
| OpCode | R3 |OpCd| B1 |     D1      | B2 |     D2     |
+--------+----+----+----+-------------+----+------------+
0        8    12   16   20            32   36           47
@end verbatim

@end table

For the complete list of all instruction format variants see the
Principles of Operation manuals.

@node s390 Aliases
@subsection Instruction Aliases
@cindex instruction aliases, s390
@cindex s390 instruction aliases

A specific bit pattern can have multiple mnemonics, for example
the bit pattern @samp{0xa7000000} has the mnemonics @samp{tmh} and
@samp{tmlh}. In addition, there are a number of mnemonics recognized by
@code{@value{AS}} that are not present in the Principles of Operation.
These are the short forms of the branch instructions, where the condition
code mask operand is encoded in the mnemonic. This is relevant for the
branch instructions, the compare and branch instructions, and the
compare and trap instructions.

For the branch instructions there are 20 condition code strings that can
be used as part of the mnemonic in place of a mask operand in the instruction
format:

@display
@multitable @columnfractions .30 .30
@headitem instruction @tab short form
@item bcr   M1,R2  @tab  b<m>r  R2
@item bc    M1,D2(X2,B2) @tab  b<m>   D2(X2,B2)
@item brc   M1,I2 @tab j<m>   I2
@item brcl  M1,I2 @tab jg<m>  I2
@end multitable
@end display

In the mnemonic for a branch instruction the condition code string <m>
can be any of the following:

@display
@multitable {nle} {jump on not zero / if not zeros}
@item o @tab jump on overflow / if ones
@item h @tab jump on A high
@item p @tab jump on plus
@item nle @tab jump on not low or equal
@item l @tab jump on A low
@item m @tab jump on minus
@item nhe @tab jump on not high or equal
@item lh @tab jump on low or high
@item ne @tab jump on A not equal B
@item nz @tab jump on not zero / if not zeros
@item e @tab jump on A equal B
@item z @tab jump on zero / if zeroes
@item nlh @tab jump on not low or high
@item he @tab jump on high or equal
@item nl @tab jump on A not low
@item nm @tab jump on not minus / if not mixed
@item le @tab jump on low or equal
@item nh @tab jump on A not high
@item np @tab jump on not plus
@item no @tab jump on not overflow / if not ones
@end multitable
@end display

For the compare and branch, and compare and trap instructions there
are 12 condition code strings that can be used as part of the mnemonic in
place of a mask operand in the instruction format:

@display
@multitable @columnfractions .40 .40
@headitem instruction @tab short form
@item crb    R1,R2,M3,D4(B4)  @tab  crb<m>    R1,R2,D4(B4)
@item cgrb   R1,R2,M3,D4(B4)  @tab  cgrb<m>   R1,R2,D4(B4)
@item crj    R1,R2,M3,I4  @tab  crj<m>    R1,R2,I4
@item cgrj   R1,R2,M3,I4  @tab  cgrj<m>   R1,R2,I4
@item cib    R1,I2,M3,D4(B4)  @tab  cib<m>    R1,I2,D4(B4)
@item cgib   R1,I2,M3,D4(B4)  @tab  cgib<m>   R1,I2,D4(B4)
@item cij    R1,I2,M3,I4  @tab  cij<m>    R1,I2,I4
@item cgij   R1,I2,M3,I4  @tab  cgij<m>   R1,I2,I4
@item crt    R1,R2,M3  @tab  crt<m>    R1,R2
@item cgrt   R1,R2,M3  @tab  cgrt<m>   R1,R2
@item cit    R1,I2,M3  @tab  cit<m>    R1,I2
@item cgit   R1,I2,M3  @tab  cgit<m>   R1,I2
@item clrb   R1,R2,M3,D4(B4)  @tab  clrb<m>   R1,R2,D4(B4)
@item clgrb  R1,R2,M3,D4(B4)  @tab  clgrb<m>  R1,R2,D4(B4)
@item clrj   R1,R2,M3,I4  @tab  clrj<m>   R1,R2,I4
@item clgrj  R1,R2,M3,I4  @tab  clgrj<m>  R1,R2,I4
@item clib   R1,I2,M3,D4(B4)  @tab  clib<m>   R1,I2,D4(B4)
@item clgib  R1,I2,M3,D4(B4)  @tab  clgib<m>  R1,I2,D4(B4)
@item clij   R1,I2,M3,I4  @tab  clij<m>   R1,I2,I4
@item clgij  R1,I2,M3,I4  @tab  clgij<m>  R1,I2,I4
@item clrt   R1,R2,M3  @tab  clrt<m>   R1,R2
@item clgrt  R1,R2,M3  @tab  clgrt<m>  R1,R2
@item clfit  R1,I2,M3  @tab  clfit<m>  R1,I2
@item clgit  R1,I2,M3  @tab  clgit<m>  R1,I2
@end multitable
@end display

In the mnemonic for a compare and branch and compare and trap instruction
the condition code string <m> can be any of the following:

@display
@multitable {nle} {jump on not zero / if not zeros}
@item h @tab jump on A high
@item nle @tab jump on not low or equal
@item l @tab jump on A low
@item nhe @tab jump on not high or equal
@item ne @tab jump on A not equal B
@item lh @tab jump on low or high
@item e @tab jump on A equal B
@item nlh @tab jump on not low or high
@item nl @tab jump on A not low
@item he @tab jump on high or equal
@item nh @tab jump on A not high
@item le @tab jump on low or equal
@end multitable
@end display

@node s390 Operand Modifier
@subsection Instruction Operand Modifier
@cindex instruction operand modifier, s390
@cindex s390 instruction operand modifier

If a symbol modifier is attached to a symbol in an expression for an
instruction operand field, the symbol term is replaced with a reference
to an object in the global offset table (GOT) or the procedure linkage
table (PLT). The following expressions are allowed:
@samp{symbol@@modifier + constant},
@samp{symbol@@modifier + label + constant}, and
@samp{symbol@@modifier - label + constant}.
The term @samp{symbol} is the symbol that will be entered into the GOT or
PLT, @samp{label} is a local label, and @samp{constant} is an arbitrary
expression that the assembler can evaluate to a constant value.

The term @samp{(symbol + constant1)@@modifier +/- label + constant2}
is also accepted but a warning message is printed and the term is
converted to @samp{symbol@@modifier +/- label + constant1 + constant2}.

@table @code
@item @@got
@itemx @@got12
The @@got modifier can be used for displacement fields, 16-bit immediate
fields and 32-bit pc-relative immediate fields. The @@got12 modifier is
synonym to @@got. The symbol is added to the GOT. For displacement
fields and 16-bit immediate fields the symbol term is replaced with
the offset from the start of the GOT to the GOT slot for the symbol.
For a 32-bit pc-relative field the pc-relative offset to the GOT
slot from the current instruction address is used.
@item @@gotent
The @@gotent modifier can be used for 32-bit pc-relative immediate fields.
The symbol is added to the GOT and the symbol term is replaced with
the pc-relative offset from the current instruction to the GOT slot for the
symbol.
@item @@gotoff
The @@gotoff modifier can be used for 16-bit immediate fields. The symbol
term is replaced with the offset from the start of the GOT to the
address of the symbol.
@item @@gotplt
The @@gotplt modifier can be used for displacement fields, 16-bit immediate
fields, and 32-bit pc-relative immediate fields. A procedure linkage
table entry is generated for the symbol and a jump slot for the symbol
is added to the GOT. For displacement fields and 16-bit immediate
fields the symbol term is replaced with the offset from the start of the
GOT to the jump slot for the symbol. For a 32-bit pc-relative field
the pc-relative offset to the jump slot from the current instruction
address is used.
@item @@plt
The @@plt modifier can be used for 16-bit and 32-bit pc-relative immediate
fields. A procedure linkage table entry is generated for the symbol.
The symbol term is replaced with the relative offset from the current
instruction to the PLT entry for the symbol.
@item @@pltoff
The @@pltoff modifier can be used for 16-bit immediate fields. The symbol
term is replaced with the offset from the start of the PLT to the address
of the symbol.
@item @@gotntpoff
The @@gotntpoff modifier can be used for displacement fields. The symbol
is added to the static TLS block and the negated offset to the symbol
in the static TLS block is added to the GOT. The symbol term is replaced
with the offset to the GOT slot from the start of the GOT.
@item @@indntpoff
The @@indntpoff modifier can be used for 32-bit pc-relative immediate
fields. The symbol is added to the static TLS block and the negated offset
to the symbol in the static TLS block is added to the GOT. The symbol term
is replaced with the pc-relative offset to the GOT slot from the current
instruction address.
@end table

For more information about the thread local storage modifiers
@samp{gotntpoff} and @samp{indntpoff} see the ELF extension documentation
@samp{ELF Handling For Thread-Local Storage}.

@node s390 Instruction Marker
@subsection Instruction Marker
@cindex instruction marker, s390
@cindex s390 instruction marker

The thread local storage instruction markers are used by the linker to
perform code optimization.

@table @code
@item :tls_load
The :tls_load marker is used to flag the load instruction in the initial
exec TLS model that retrieves the offset from the thread pointer to a
thread local storage variable from the GOT.
@item :tls_gdcall
The :tls_gdcall marker is used to flag the branch-and-save instruction to
the __tls_get_offset function in the global dynamic TLS model.
@item :tls_ldcall
The :tls_ldcall marker is used to flag the branch-and-save instruction to
the __tls_get_offset function in the local dynamic TLS model.
@end table

For more information about the thread local storage instruction marker
and the linker optimizations see the ELF extension documentation
@samp{ELF Handling For Thread-Local Storage}.

@node s390 Literal Pool Entries
@subsection Literal Pool Entries
@cindex literal pool entries, s390
@cindex s390 literal pool entries

A literal pool is a collection of values. To access the values a pointer
to the literal pool is loaded to a register, the literal pool register.
Usually, register %r13 is used as the literal pool register
(@ref{s390 Register}). Literal pool entries are created by adding the
suffix :lit1, :lit2, :lit4, or :lit8 to the end of an expression for an
instruction operand. The expression is added to the literal pool and the
operand is replaced with the offset to the literal in the literal pool.

@table @code
@item :lit1
The literal pool entry is created as an 8-bit value. An operand modifier
must not be used for the original expression.
@item :lit2
The literal pool entry is created as a 16 bit value. The operand modifier
@@got may be used in the original expression. The term @samp{x@@got:lit2}
will put the got offset for the global symbol x to the literal pool as
16 bit value.
@item :lit4
The literal pool entry is created as a 32-bit value. The operand modifier
@@got and @@plt may be used in the original expression. The term
@samp{x@@got:lit4} will put the got offset for the global symbol x to the
literal pool as a 32-bit value. The term @samp{x@@plt:lit4} will put the
plt offset for the global symbol x to the literal pool as a 32-bit value.
@item :lit8
The literal pool entry is created as a 64-bit value. The operand modifier
@@got and @@plt may be used in the original expression. The term
@samp{x@@got:lit8} will put the got offset for the global symbol x to the
literal pool as a 64-bit value. The term @samp{x@@plt:lit8} will put the
plt offset for the global symbol x to the literal pool as a 64-bit value.
@end table

The assembler directive @samp{.ltorg} is used to emit all literal pool
entries to the current position.

@node s390 Directives
@section Assembler Directives

@code{@value{AS}} for s390 supports all of the standard ELF
assembler directives as outlined in the main part of this document.
Some directives have been extended and there are some additional
directives, which are only available for the s390 @code{@value{AS}}.

@table @code
@cindex @code{.insn} directive, s390
@item .insn
This directive permits the numeric representation of an instructions
and makes the assembler insert the operands according to one of the
instructions formats for @samp{.insn} (@ref{s390 Formats}).
For example, the instruction @samp{l %r1,24(%r15)} could be written as
@samp{.insn rx,0x58000000,%r1,24(%r15)}.
@cindex @code{.short} directive, s390
@cindex @code{.long} directive, s390
@cindex @code{.quad} directive, s390
@item .short
@itemx .long
@itemx .quad
This directive places one or more 16-bit (.short), 32-bit (.long), or
64-bit (.quad) values into the current section. If an ELF or TLS modifier
is used only the following expressions are allowed:
@samp{symbol@@modifier + constant},
@samp{symbol@@modifier + label + constant}, and
@samp{symbol@@modifier - label + constant}.
The following modifiers are available:
@table @code
@item @@got
@itemx @@got12
The @@got modifier can be used for .short, .long and .quad. The @@got12
modifier is synonym to @@got. The symbol is added to the GOT. The symbol
term is replaced with offset from the start of the GOT to the GOT slot for
the symbol.
@item @@gotoff
The @@gotoff modifier can be used for .short, .long and .quad. The symbol
term is replaced with the offset from the start of the GOT to the address
of the symbol.
@item @@gotplt
The @@gotplt modifier can be used for .long and .quad. A procedure linkage
table entry is generated for the symbol and a jump slot for the symbol
is added to the GOT. The symbol term is replaced with the offset from the
start of the GOT to the jump slot for the symbol.
@item @@plt
The @@plt modifier can be used for .long and .quad. A procedure linkage
table entry us generated for the symbol. The symbol term is replaced with
the address of the PLT entry for the symbol.
@item @@pltoff
The @@pltoff modifier can be used for .short, .long and .quad. The symbol
term is replaced with the offset from the start of the PLT to the address
of the symbol.
@item @@tlsgd
@itemx @@tlsldm
The @@tlsgd and @@tlsldm modifier can be used for .long and .quad. A
tls_index structure for the symbol is added to the GOT. The symbol term is
replaced with the offset from the start of the GOT to the tls_index structure.
@item @@gotntpoff
@itemx @@indntpoff
The @@gotntpoff and @@indntpoff modifier can be used for .long and .quad.
The symbol is added to the static TLS block and the negated offset to the
symbol in the static TLS block is added to the GOT. For @@gotntpoff the
symbol term is replaced with the offset from the start of the GOT to the
GOT slot, for @@indntpoff the symbol term is replaced with the address
of the GOT slot.
@item @@dtpoff
The @@dtpoff modifier can be used for .long and .quad. The symbol term
is replaced with the offset of the symbol relative to the start of the
TLS block it is contained in.
@item @@ntpoff
The @@ntpoff modifier can be used for .long and .quad. The symbol term
is replaced with the offset of the symbol relative to the TCB pointer.
@end table

For more information about the thread local storage modifiers see the
ELF extension documentation @samp{ELF Handling For Thread-Local Storage}.

@cindex @code{.ltorg} directive, s390
@item .ltorg
This directive causes the current contents of the literal pool to be
dumped to the current location (@ref{s390 Literal Pool Entries}).

@cindex @code{.machine} directive, s390
@item .machine string
This directive allows you to change the machine for which code is
generated.  @code{string} may be any of the @code{-march=} selection
options (without the -march=), @code{push}, or @code{pop}.
@code{.machine push} saves the currently selected cpu, which may be
restored with @code{.machine pop}.  Be aware that the cpu string has
to be put into double quotes in case it contains characters not
appropriate for identifiers.  So you have to write @code{"z9-109"}
instead of just @code{z9-109}.

@cindex @code{.machinemode} directive, s390
@item .machinemode string
This directive allows to change the architecture mode for which code
is being generated.  @code{string} may be @code{esa}, @code{zarch},
@code{zarch_nohighgprs}, @code{push}, or @code{pop}.
@code{.machinemode zarch_nohighgprs} can be used to prevent the
@code{highgprs} flag from being set in the ELF header of the output
file.  This is useful in situations where the code is gated with a
runtime check which makes sure that the code is only executed on
kernels providing the @code{highgprs} feature.
@code{.machinemode push} saves the currently selected mode, which may
be restored with @code{.machinemode pop}.
@end table

@node s390 Floating Point
@section Floating Point
@cindex floating point, s390
@cindex s390 floating point

The assembler recognizes both the @sc{ieee} floating-point instruction and
the hexadecimal floating-point instructions. The floating-point constructors
@samp{.float}, @samp{.single}, and @samp{.double} always emit the
@sc{ieee} format. To assemble hexadecimal floating-point constants the
@samp{.long} and @samp{.quad} directives must be used.