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PowerPC development

The following documentation discusses cross-development with the PowerPC targets.

- Compiling for PowerPC targets

- Floating-point subroutines for PowerPC targets
- Preprocessor macro for PowerPC targets

- Assembler options for PowerPC targets
- Debugging PowerPC targets

- The stack frame
- Argument passing
- Function return values

Cross-development tools in the GNUPro Toolkit are normally installed with names that reflect the target machine, so that you can install more than one set of tools in the same binary directory. The target name, constructed with the `--target` option to `configure` , is used as a prefix to the program name. For example, the compiler for the PowerPC`` (`gcc` in native configurations) is called, depending on which configuration you have installed, by `powerpc-eabi-gcc`.
The following processors are supported for the PowerPC targets.

`403Gx`

`603(e)`

`505`

`604`

`601`

`604(e)`

`602`

`821`

`603`

`860`

Compiling for PowerPC targets
The PowerPC target family toolchain controls variances in code generation directly from the command line.
When you run `gcc`, you can use command-line options to choose whether to take advantage of the extra PowerPC machine instructions, and whether to generate code for hardware or software floating point.
When you run `gcc`, you can use command-line options to choose machine-specific details.
The following `-m` options are defined for the PowerPC.

`-mpower`
`-mno-power`
`-mpower2`
`-mno-power2`
`-mpowerpc`
`-mno-powerpc`
`-mpowerpc-gpopt`
`-mno-powerpc-gpopt`
`-mpowerpc-gfxopt`
`-mno-powerpc-gfxopt`

GNU CC supports two related instruction set architectures for the IBM RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the `rios` chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MCP8xx and the IBM 4xx microprocessors. The PowerPC architecture defines 64-bit instructions, but they are not supported by any current processors.

Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.

You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GNU CC. Specifying the `-mcpu=cpu_type` overrides the specification of these options.

We recommend you use the `-mcpu=cpu_type` option rather than any of these options.
The `-mpower` option allows GNU CC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying `-mpower2` implies `-power` and also allows GNU CC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.
The `-mpowerpc` option allows GNU CC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying `-mpowerpc-gpopt` implies `-mpowerpc` and also allows GNU CC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying `-mpowerpc-gfxopt` implies `-mpowerpc` and also allows GNU CC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.
If you specify both `-mno-power` and `-mno-powerpc` , GNU CC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both `-mpower` and `-mpowerpc` permits GNU CC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

`-mnew-mnemonics`
`-mold-mnemonics`

Select which mnemonics to use in the generated assembler code.
`-mnew-mnemonics` requests output that uses the assembler mnemonics defined for the PowerPC architecture, while `-mold-mnemonics` requests the assembler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GNU CC uses that mnemonic irrespective of which of these options is specified.
PowerPC assemblers support both the old and new mnemonics, as will later POWER assemblers. Current POWER assemblers only support the old mnemonics. Specify `-mnew-mnemonics` if you have an assembler that supports them, otherwise specify `-mold-mnemonics` .
The default value of these options depends on how GNU CC was configured. Specifying `-mcpu=cpu_type` sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either `-mnew-mnemonics` or `-mold-mnemonics` , but should instead accept the default.

`-mcpu=cpu_type`

Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type, `cpu_type`. Supported values for `cpu_type` are `rs6000`, `rios1`, `rios2`, `rsc`, `601`, `602`, `603`, `603e`, `604`, `604e`, `620`, `power`, `power2`, `powerpc`, `403`, `505`, `801`, `821`, `823`, `860` and `common`.

The `-mcpu=power`, `-mcpu=power2`, and `-mcpu=powerpc` specify generic POWER, POWER2 and pure PowerPC (i.e., not MPC601) architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.
Specifying `-mcpu=rios1`, `-mcpu=rios2`, `-mcpu=rsc`, `-mcpu=power`, or `-mcpu=power2` enables the `-mpower` option and disables the `-mpowerpc` option; `-mcpu=601` enables both the `-mpower` and `-mpowerpc` options; `-mcpu=602`, `-mcpu=603`, `-mcpu=603e`, `-mcpu=604`, `-mcpu=620` ; `-mcpu=403`, `-mcpu=505`, `-mcpu=821`, `-mcpu=860` and `-mcpu=powerpc` enable the `-mpowerpc` option and disable the `-mpower` option; `-mcpu=common` disables both the `-mpower` and `-mpowerpc` options.
IBM AIX versions 4 or greater selects `-mcpu=common` by default, so that code will operate on all members of the IBM RS/6000 and PowerPC families. In that case, GNU CC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. GNU CC assumes a generic processor model for scheduling purposes.
Specifying `-mcpu=rios1`, `-mcpu=rios2`, `-mcpu=rsc`, `-mcpu=power`, or `-mcpu=power2` also disables the `new-mnemonics` option.
Specifying `-mcpu=601` , `-mcpu=602`, `-mcpu=603`, `-mcpu=603e`, `-mcpu=604` , `-mcpu=620`, `-mcpu=403`, or `-mcpu=powerpc` also enables the `new-mnemonics` option.
Specifying `-mcpu=403` , `-mcpu=821`, or `-mcpu=860` also enables the `-msoft-float` option.

`-mtune=cpu_type`

Set the instruction scheduling parameters for machine type, `cpu_type`, but do not set the architecture type, register usage, choice of mnemonics like `-mcpu=cpu_type` would. The same values for `cpu_type` are used for `-mtune=cpu_type` as for `-mcpu=cpu_type` . The `-mtune=cpu_type` option overrides the `-mcpu=cpu_type` option in terms of instruction scheduling parameters.

`-mfull-toc`
`-mno-fp-in-toc`
`-mno-sum-in-toc`
`-mminimal-toc`

Modify generation of the TOC (Table Of Contents), which is created for every executable file. The `-mfull-toc` option is selected by default. In that case, GNU CC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GNU CC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.

If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the `-mno-fp-in-toc` and `-mno-sum-in-toc` options.

`-mno-fp-in-toc` prevents GNU CC from putting floating-point constants in the TOC and `-mno-sum-in-toc` forces GNU CC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC.

You may specify one or both of these options. Each causes GNU CC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify `-mminimal-toc` instead. This option causes GNU CC to make only one TOC entry for every file. When you specify this option, GNU CC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.

`-msoft-float`
`-mhard-float`

Generate code that does not use or does use the floating-point register set. Software floating point emulation is provided if you use the `-msoft-float` option, and pass the option to GNU CC when linking.

`-mmultiple`
`-mno-multiple`

Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use `-mmultiple` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.

`-mstring`
`-mno-string`

Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems.

Warning:

Do not use `-mstring` on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.

`-mupdate`
`-mno-update`

Generate code that uses (or does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default.

If you use `-mno-update` , there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.

`-mfused-madd`
`-mno-fused-madd`

Generate code that uses (does not use) the floating point multiply and accumulate instructions. These instructions are generated by default if hardware floating is used.

`-mno-bit-align`
`-mbit-align`

On System V.4 and embedded PowerPC systems do not and do force structures and unions containing bit fields aligned to the base type of the bit field. For example, by default a structure containing nothing but 8 unsigned bitfields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using `-mno-bit-align`, the structure would be aligned to a 1 byte boundary and be one byte in size.

`-mno-strict-align`
`-mstrict-align`

On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.

`-mrelocatable`
`-mno-relocatable`

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. If you use `-mrelocatable` on any module, all objects linked together must be compiled with `-mrelocatable` or `-mrelocatable-lib`.

`-mrelocatable-lib`
`-mno-relocatable-lib`

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime. Modules compiled with `-mreloctable-lib` can be linked with either modules compiled without `-mrelocatable` and `-mrelocatable-lib` or with modules compiled with the `-mrelocatable` options.

`-mno-toc`
`-mtoc`

On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

`-mno-traceback`
`-mtraceback`

On embedded PowerPC systems do not (do) generate a trace-back tag before the start of the function. This tag can be used by the debugger to identify where the start of a function is.

`-mlittle`
`-mlittle-endian`

On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The `-mlittle-endian` option is the same as `-mlittle` .

`-mbig`
`-mbig-endian`

On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The `-mbig-endian` option is the same as `-mbig` .

`-mcall-sysv`

On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using `powerpc-*-eabiaix` .

`-mcall-sysv-eabi`

Specify both `-mcall-sysv` and `-meabi` options.

`-mcall-sysv-noeabi`

Specify both `-mcall-sysv` and `-mnoeabi` options.

`-mcall-aix`

On System V.4 and embedded PowerPC systems compile code using calling conventions that are similar to those used on AIX. This is the default if you configured GCC using `powerpc-*-eabiaix` .

`-mcall-solaris`

On System V.4 and embedded PowerPC systems, compile code for the Solaris operating system.

`-mcall-linux`

On System V.4 and embedded PowerPC systems, compile code for the Linux operating system.

`-mprototype`
`-mno-prototype`

On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (`CR`) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments.

With `-mprototype`, only calls to prototyped variable argument functions will set or clear the bit.

`-msim`

On embedded PowerPC systems, assume that the startup module is called `sim-crt0.o` and the standard C libraries are `libsim.a` and `libc.a`. This is default for `powerpc-*-eabisim` configurations.

`-mmvme`

On embedded PowerPC systems, assume that the startup module is called `mvme-crt0.o` and the standard C libraries are `libmvme.a` and `libc.a` .

`-memb`

On embedded PowerPC systems, set the `PPC_EMB` bit in the ELF flags header to indicate that `eabi` extended relocations are used.

`-mads`

On embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libads.a` and `libc.a` .

`-myellowknife`

On embedded PowerPC systems, assume that the startup module is called `crt0.o` and ` `libyk.a` and ` `libc.a` are the standard C libraries.

`-meabi`
`-mno-eabi`

On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (EABI) which is a set of modifications to the System V.4 specifications. Selecting `-meabi` means that the stack is aligned to an 8 byte boundary, a function, `__eabi`, is called to from `main` to set up the EABI environment, and the `-msdata` option can use both `r2` and `r13` to point to two separate small data areas.

Selecting `-mno-eabi` means that the stack is aligned to a 16 byte boundary, do not call an initialization function from main, and the
`-msdata` option will only use `r13` to point to a single small data area. The `-meabi` option is on by default if you configured GCC using one of the `powerpc*-*-eabi*` options.

`-msdata=eabi`

On System V.4 and embedded PowerPC systems, put small initialized const global and static data in the `.sdata2` section, which is pointed to by register `r2` . Put small initialized non-const global and static data in the `.sdata` section, which is pointed to by register `r13` . Put small uninitialized global and static data in the `.sbss` section, which is adjacent to the `.sdata` section. The `-msdata=eabi` option is incompatible with the `-mrelocatable` option. The `-msdata=eabi` option also sets the `-memb` option.

`-msdata=sysv`

On System V.4 and embedded PowerPC systems, put small global and static data in the `.sdata` section, which is pointed to by register `r13` . Put small uninitialized global and static data in the `.sbss` section, which is adjacent to the `.sdata` section.

The `-msdata=sysv` option is incompatible with the `-mrelocatable` option.

`-msdata=default`
`-msdata`

On System V.4 and embedded PowerPC systems, if `-meabi` is used, compile code the same as `-msdata=eabi`, otherwise compile code the same as `-msdata=sysv` .

`-msdata-data`

On System V.4 and embedded PowerPC systems, put small global and static data in the `.sdata` section. Put small uninitialized global and static data in the `.sbss` section. Do not use register `r13` to address small data however.

This is the default behavior unless other `-msdata` options are used.

`-msdata=none`
`-mno-sdata`

On embedded PowerPC systems, put all initialized global and static data in the `.data` section, and all uninitialized data in the `.bss` section.

`-G` `num`

On embedded PowerPC systems, put global and static items less than or equal to `num` bytes into the small data or bss sections instead of the normal data or bss section. By default, num is 8. The ` `-G` num switch is also passed to the linker. All modules should be compiled with the same `-G` `num` value.

`-mregnames`
`-mno-regnames`

On System V.4 and embedded PowerPC systems, do (do not) emit register names in the assembly language output using symbolic forms.

Floating-point subroutines for PowerPC
The following two kinds of floating point subroutines are useful with the GNU compiler.

- Software implementations of the basic functions (floating-point multiply, divide, add, subtract), for use when there is no hardware floating-point support.
- General-purpose mathematical subroutines, included with implementation of the standard C mathematical subroutine library. See Mathematical functions (`math.h`) in GNUPro Math Library in GNUPro Libraries.

Preprocessor macro for PowerPC targets
GCC defines the preprocessor macro, `__powerpc-eabi__`, for the PowerPC configurations.

Assembler options for PowerPC targets

To use the GNU assembler to assemble GCC output, configure GCC with the `--with-gnu-as` switch or with the `-mgas` option.

`-mgas`

Compile using the GNU assembler, `gas`, to assemble GCC output.

`-Wa`

If you invoke the GNU assembler through the GNU C compiler (version 2), you can use the `-Wa` option to pass arguments through to the assembler. One common use of this option is to exploit the assembler s listing features.

Assembler arguments that you specify with GCC,`-Wa` must be separated from each other (and the `-Wa`) by commas, like the options, `-alh` and `-L`, in the following example input, separate from `-Wa`.

    powerpc-eabi-gcc -c -g -O -Wa,-alh, -L file.c

`-L`

The additional assembler option, `-L`, preserves local labels, which may make the listing output more intelligible to humans.
For example, in the following commandline, the assembler option,`-ahl`, requests a listing with interspersed high-level language and assembly language.

    powerpc-eabi-gcc -c -g -O -Wa,-alh,-L file.c

`-L` preserves local labels, while the compiler debugging option, `-g`, gives the assembler the necessary debugging information.
Use the following options to enable listing output from the assembler. The letters after `-a` may be combined into one option, such as `-al` .

`-a`

By itself, -a requests listings of high-level language source, assembly language, and symbols.

`-ah`

Requests a high-level language listing.

`-al`

Request an output-program assembly listing.

`-as`

Requests a symbol table listing.

`-ad`

Omits debugging directives from listing. High-level listings require a compiler debugging option like `-g`, and assembly listings (such as `-al`) requested.

Use the following listing-control assembler directives to control the appearance of the listing output (if you do not request listing output with one of the `-a` options, the following listing-control directives have no effect).

`.list`

Turn on listings for further input.

`.nolist`

Turn off listings for further input.

`.psize linecount, columnwidth`

Describe the page size for your output (the default is `60, 200`). `gas` generates form feeds after printing each group of `linecount` lines. To avoid these automatic form feeds, specify `0` as `linecount`. The variable input for `columnwidth` uses the same descriptive option.

`.eject`

Skip to a new page (issue a form feed).

`.title`

Use as the title (this is the second line of the listing output, directly after the source file name and page number) when generating assembly listings.

`.sbttl`

Use as the subtitle (this is the third line of the listing output, directly after the title line) when generating assembly listings.

`-an`

Turn off all forms processing.

Debugging PowerPC targets
The PowerPC-configured GNU debugger, GDB, is called by `powerpc-eabi-gdb`.
GDB needs to know the following specifications to talk to PowerPC targets.

- Specifications for what you want to use one, such as `target remote`, `gdb` s generic debugging protocol.
- Specifications for what serial device connects your PowerPC board (the first serial device available on your host is the default).
- Specifications for what speed to use over the serial device.

Use the following GDB commands to specify the connection to your target board.

`target powerpc serial-device`

To run a program on the board, start up GDB with the name of your program as the argument. To connect to the board, use the command, `target interface serial-device`, where `interface` is an interface from the previous list of specifications and `serial-device` is the name of the serial port connected to the board. If the program has not already been downloaded to the board, you may use the `load` command to download it. You can then use all the usual `gdb` commands. For example, the following sequence connects to the target board through a serial port, and loads and runs programs, designated here as `prog`, through the debugger.

    (gdb) target powerpc com1

    ...

    breakinst () ../sparc-stub.c:975

    975     }

    (gdb) s

    main    ()   hello.c:50

    50       writer(1,    "Got to here\n");

    (gdb)

`target powerpc` `hostname : portnumber`

You can specify a TCP/IP connection instead of a serial port, using the syntax, `hostname : portnumber` (assuming your board, designated here as `hostname`, is connected, for instance, to use a serial line, designated by `portnumber`, managed by a terminal concentrator).

GDB also supports `set remotedebug` `n`. You can see some debugging information about communications with the board by setting the variable, `remotedebug`.

The stack frame
The following information applies to the stack frame for the PowerPC.

- The stack grows downwards from high addresses to low addresses.
- A leaf function need not allocate a stack frame if it does not need one.
- A frame pointer need not be allocated.
- The stack pointer shall always be aligned to 4 byte boundaries.
- The register save area shall be aligned to a 4 byte boundary.

Stack frames for functions taking a fixed number of arguments use the definitions in the following chart. FP points to the same location as SP.
Stack frames for functions that take a variable number of arguments use the following definitions.

Argument passing
The following table shows the general purpose registers, floating point registers, and the stack frame offset.

General Purpose Registers | Floating-Point Registers | Stack Frame Offset
--- | --- | ---
`r3: c` | `f1: ff` | `08: ptr to t`
`r4: d` | `f2: gg` | `0c: (padding)`
`r5: e` | `f3: hh` | `10: nn(lo)`
`r6: f` | `f4: ii` | `14: nn(hi)`
`r7: g` | `f5: jj` |
`r8: h` | `f6: kk` |
`r9: ptr to ld` | `f7: ll` |
`r10: ptr to s` | `f8: mm` |

Function return values
Integers, floating point values, and aggregates of 8 bytes or less are returned in register `r0` (and `r1` if necessary).
Aggregates larger than 8 bytes are returned by having the caller pass the address of a buffer to hold the value in `r0` as an invisible first argument. All arguments are then shifted down by one. The address of this buffer is returned in `r0` .

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