📄 m68k-configuration.html
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><H1
><A
NAME="M68K-CONFIGURATION"
></A
>Configuration</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN18880"
></A
><H2
>Name</H2
>Options -- Configuring the M68K Architectural Package</DIV
><DIV
CLASS="REFSECT1"
><A
NAME="M68K-CONFIG-LOAD"
></A
><H2
>Loading and Unloading the Package</H2
><P
>The M68K architectural HAL package <CODE
CLASS="VARNAME"
>CYGPKG_HAL_M68K</CODE
>
should be loaded automatically when eCos is configured for M68K-based
target hardware. It should never be necessary to load this package
explicitly. Unloading the package should only happen as a side effect
of switching target hardware. <CODE
CLASS="VARNAME"
>CYGPKG_HAL_M68K</CODE
>
serves primarily as a container for lower-level HALs and has only a
small number of configuration options.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="M68K-CONFIG-STACKS"
></A
><H2
>Stacks</H2
><P
>By default the architectural HAL provides a single block of memory to
act as both the startup stack and the interrupt stack. The variant,
processor or platform HAL may override this. For example if there are
several banks of RAM with different performance characteristics it may
be desirable to place the interrupt stack in fast RAM rather than in
ordinary RAM.
</P
><P
>The assembler startup code sets the stack pointer to the startup stack
before switching to C code. This stack used for all HAL
initialization, running any C++ static constructors defined either by
eCos or by the application, and the <CODE
CLASS="FUNCTION"
>cyg_start</CODE
>
entry point. In configurations containing the eCos kernel
<CODE
CLASS="FUNCTION"
>cyg_start</CODE
> will enable interrupts, activate the
scheduler and threads will then run on their own stacks. In non-kernel
single-threaded applications the whole system continues to run on the
startup stack.
</P
><P
>When an interrupt occurs the default behaviour is to switch to a
separate interrupt stack. This behaviour is controlled by the common
HAL configuration option
<CODE
CLASS="VARNAME"
>CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK</CODE
>.
It reduces the stack requirements of all threads in the system, at the
cost of some extra instructions during interrupt handling. In kernel
configurations the startup stack is no longer used once the scheduler
starts running so its memory can be reused for the interrupt stack. To
handle the possibility of nested interrupts the interrupt handling
code will detect if it is already on the interrupt stack, so in
non-kernel configurations it is also safe to use the same area of
memory for both startup and interrupt stacks. This leads to the
following scenarios:
</P
><P
></P
><OL
TYPE="1"
><LI
><P
>If interrupt stacks are enabled via
<CODE
CLASS="VARNAME"
>CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK</CODE
>
and the interrupt stack is not provided by the variant, processor or
platform HAL then a single block of memory will be used for both
startup and interrupt stacks. The size of this block is determined by
the common HAL configuration option
<CODE
CLASS="VARNAME"
>CYGNUM_HAL_COMMON_INTERRUPTS_STACK_SIZE</CODE
>, with a
default value
<CODE
CLASS="VARNAME"
>CYGNUM_HAL_DEFAULT_INTERRUPT_STACK_SIZE</CODE
> provided by
the M68K architectural HAL.
</P
></LI
><LI
><P
>If the use of an interrupt stack is disabled then the M68K
architectural HAL will provide just the startup stack, unless this is
done by the variant, processor or platform HAL. The size of the
startup stack is controlled by
<CODE
CLASS="VARNAME"
>CYGNUM_HAL_M68K_STARTUP_STACK_SIZE</CODE
>.
</P
></LI
><LI
><P
>Otherwise the interrupt and/or startup stacks are provided by other
packages and it is up to those packages to provide configuration
options for setting the sizes.
</P
></LI
></OL
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="M68K-CONFIG-FPU"
></A
><H2
>Floating Point Support</H2
><P
>There are many variants of the basic M68K architecture. Some of these
have hardware floating point support. Originally this came in the form
of a separate 68881 coprocessor, but with modern variants it will be
part of the main processor chip. If the processor does not have
hardware floating point then software emulation will be used instead.
</P
><P
>If the processor on the target hardware has a floating point unit then
the variant or processor HAL will implement the CDL interface
<CODE
CLASS="VARNAME"
>CYGINT_HAL_M68K_VARIANT_FPU</CODE
>. This allows the
architectural HAL and other packages to do the right thing on
different hardware.
</P
><P
>Saving and restoring hardware floating point context increases
interrupt and dispatch latency, code size, and data size. If the
application does not actually use floating point then these overheads
are unnecessary, and can be suppressed by disabling the configuration
option <CODE
CLASS="VARNAME"
>CYGIMP_HAL_M68K_FPU_SAVE</CODE
>. Some applications
do use floating point but only in one thread. In that scenario it is
also unnecessary to save the floating point context during interrupts
and context switches, so the configuration option can be disabled.
</P
><P
>The exact behaviour of the hardware floating point unit is determined
by the floating point control register <CODE
CLASS="VARNAME"
>%fpcr</CODE
>. By
default this is initialized to 0, giving IEE754 standard behaviour,
but another initial value can be specified using the configuration
option <CODE
CLASS="VARNAME"
>CYGNUM_HAL_M68K_FPU_CR_DEFAULT</CODE
>. For details
of the various bits in this control register see appropriate hardware
documentation. eCos assumes that the control register does not change
on a per-thread basis and hence the register is not saved or restored
during interrupt handling or a context switch.
</P
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>Warning</B
></TD
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><TR
><TD
ALIGN="LEFT"
><P
>At the time of writing eCos has not run on an M68K processor with
hardware floating point so the support for this is untested.
</P
></TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="M68K-CONFIG-OTHER"
></A
><H2
>Other Options</H2
><P
>There are configuration options to change the compiler flags used for
building this packages.
The M68K architectural HAL package does not define any other
configuration options that can be manipulated by the user. It does
define a number of interfaces such as
<CODE
CLASS="VARNAME"
>CYGINT_HAL_M68K_USE_STANDARD_PLATFORM_STUB_SUPPORT</CODE
>
which can be used by lower levels of the M68K HAL hierarchy to enable
certain functionality within the architectural package. Usually these
are of no interest to application developers.
</P
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