HAL Structure

In order to write an eCos HAL it's a good idea to have at least a passing understanding of how the HAL interacts with the rest of the system.

HAL Classes

The eCos HAL consists of four HAL sub-classes. This table gives a brief description of each class and partly reiterates the description in Chapter 2. The links refer to the on-line CVS tree (specifically to the sub-HALs used by the PowerPC MBX target).

HAL type	Description	Functionality Overview
Common HAL (hal/common)	Configuration options and functionality shared by all HALs.	Generic debugging functionality, driver API, eCos/ROM monitor calling interface, and tests.
Architecture HAL (hal/<architecture>/arch)	Functionality specific to the given architecture. Also default implementations of some functionality which can be overridden by variant or platform HALs.	Architecture specific debugger functionality (handles single stepping, exception-to-signal conversion, etc.), exception/interrupt vector definitions and handlers, cache definition and control macros, context switching code, assembler functions for early system initialization, configuration options, and possibly tests.
Variant HAL (hal/<architecture>/<variant>)	Some CPU architectures consist of a number variants, for example MIPS CPUs come in both 32 and 64 bit versions, and some variants have embedded features additional to the CPU core.	Variant extensions to the architecture code (cache, exception/interrupt), configuration options, possibly drivers for variant on-core devices, and possibly tests.
Platform HAL (hal/<architecture>/<platform>)	Contains functionality and configuration options specific to the platform.	Early platform initialization code, platform memory layout specification, configuration options (processor speed, compiler options), diagnostic IO functions, debugger IO functions, platform specific extensions to architecture or variant code (off-core interrupt controller), and possibly tests.
Auxiliary HAL (hal/<architecture>/<module>)	Some variants share common modules on the core. Motorola's PowerPC QUICC is an example of such a module.	Module specific functionality (interrupt controller, simple device drivers), possibly tests.

File Descriptions

Listed below are the files found in various HALs, with a short description of what each file contains. When looking in existing HALs beware that they do not necessarily follow this naming scheme. If you are writing a new HAL, please try to follow it as closely as possible. Still, no two targets are the same, so sometimes it makes sense to use additional files.

Common HAL

File	Description
`include/dbg-thread-syscall.h`	Defines the thread debugging syscall function. This is used by the ROM monitor to access the thread debugging API in the RAM application. .
`include/dbg-threads-api.h`	Defines the thread debugging API. .
`include/drv_api.h`	Defines the driver API.
`include/generic-stub.h`	Defines the generic stub features.
`include/hal_if.h`	Defines the ROM/RAM calling interface API.
`include/hal_misc.h`	Defines miscellaneous helper functions shared by all HALs.
`include/hal_stub.h`	Defines eCos mappings of GDB stub features.
`src/dbg-threads-syscall.c`	Thread debugging implementation.
`src/drv_api.c`	Driver API implementation. Depending on configuration this provides either wrappers for the kernel API, or a minimal implementation of these features. This allows drivers to be written relying only on HAL features.
`src/dummy.c`	Empty dummy file ensuring creation of libtarget.a.
`src/generic-stub.c`	Generic GDB stub implementation. This provides the communication protocol used to communicate with GDB over a serial device or via the network.
`src/hal_if.c`	ROM/RAM calling interface implementation. Provides wrappers from the calling interface API to the eCos features used for the implementation.
`src/hal_misc.c`	Various helper functions shared by all platforms and architectures.
`src/hal_stub.c`	Wrappers from eCos HAL features to the features required by the generic GDB stub.
`src/stubrom/stubrom.c`	The file used to build eCos GDB stub images. Basically a cyg_start function with a hard coded breakpoint.
`src/thread-packets.c`	More thread debugging related functions.
`src/thread-pkts.h`	Defines more thread debugging related function.

Architecture HAL

Some architecture HALs may add extra files for architecture specific serial drivers, or for handling interrupts and exceptions if it makes sense.

Note that many of the definitions in these files are only conditionally defined - if the equivalent variant or platform headers provide the definitions, those override the generic architecture definitions.

File	Description
`include/arch.inc`	Various assembly macros used during system initialization.
`include/basetype.h`	Endian, label, alignment, and type size definitions. These override common defaults in CYGPKG_INFRA.
`include/hal_arch.h`	Saved register frame format, various thread, register and stack related macros.
`include/hal_cache.h`	Cache definitions and cache control macros.
`include/hal_intr.h`	Exception and interrupt definitions. Macros for configuring and controlling interrupts. eCos real-time clock control macros.
`include/hal_io.h`	Macros for accessing IO devices.
`include/<arch>_regs.h`	Architecture register definitions.
`include/<arch>_stub.h`	Architecture stub definitions. In particular the register frame layout used by GDB. This may differ from the one used by eCos.
`include/<arch>.inc`	Architecture convenience assembly macros.
`src/<arch>.ld`	Linker macros.
`src/context.S`	Functions handling context switching and setjmp/longjmp.
`src/hal_misc.c`	Exception and interrupt handlers in C. Various other utility functions.
`src/hal_mk_defs.c`	Used to export definitions from C header files to assembler header files.
`src/hal_intr.c`	Any necessary interrupt handling functions.
`src/<arch>stub.c`	Architecture stub code. Contains functions for translating eCos exceptions to UNIX signals and functions for single-stepping.
`src/vectors.S`	Exception, interrupt and early initialization code.

Variant HAL

Some variant HALs may add extra files for variant specific serial drivers, or for handling interrupts/exceptions if it makes sense.

Note that these files may be mostly empty if the CPU variant can be controlled by the generic architecture macros. The definitions present are only conditionally defined - if the equivalent platform headers provide the definitions, those override the variant definitions.

File	Description
`include/var_arch.h`	Saved register frame format, various thread, register and stack related macros.
`include/var_cache.h`	Cache related macros.
`include/var_intr.h`	Interrupt related macros.
`include/var_regs.h`	Extra register definitions for the CPU variant.
`include/variant.inc`	Various assembly macros used during system initialization.
`src/var_intr.c`	Interrupt functions if necessary.
`src/var_misc.c`	hal_variant_init function and any necessary extra functions.
`src/variant.S`	Interrupt handler table definition.
`src/<arch>_<variant>.ld`	Linker macros.

Platform HAL

Extras files may be added for platform specific serial drivers. Extra files for handling interrupts and exceptions will be present if it makes sense.

File	Description
`include/hal_diag.h`	Defines functions used for HAL diagnostics output. This would normally be the ROM calling interface wrappers, but may also be the low-level IO functions themselves, saving a little overhead.
`include/platform.inc`	Platform initialization code. This includes memory controller, vectors, and monitor initialization. Depending on the architecture, other things may need defining here as well: interrupt decoding, status register initialization value, etc.
`include/plf_cache.h`	Platform specific cache handling.
`include/plf_intr.h`	Platform specific interrupt handling.
`include/plf_io.h`	PCI IO definitions and macros. May also be used to override generic HAL IO macros if the platform endianness differs from that of the CPU.
`include/plf_stub.h`	Defines stub initializer and board reset details.
`src/hal_diag.c`	May contain the low-level device drivers. But these may also reside in plf_stub.c
`src/platform.S`	Memory controller setup macro, and if necessary interrupt springboard code.
`src/plf_misc.c`	Platform initialization code.
`src/plf_mk_defs.c`	Used to export definitions from C header files to assembler header files.
`src/plf_stub.c`	Platform specific stub initialization and possibly the low-level device driver.

The platform HAL also contains files specifying the platform's memory layout. These files are located in include/pkgconf.

Auxiliary HAL

Auxiliary HALs contain whatever files are necessary to provide the required functionality. There are no predefined set of files required in an auxiliary HAL.