The <iohw.h> header is intended to be delivered from the compiler tool vendor for the specific CPU / platform type.
An implementation may use any intrinsic functionality provided by the compiler / tool vendor.
For the same reason is will also often be the compiler / tool vendor which have the last word about how ioreg_designators for the I/O registers is defined with the specific tool / platform.

This may sound complicated but in practice it is usually rather straightforward for most cases.

Consider these simple examples for a processor with memory mapped I/O and one single address bus range.

Implementation example 1

The example illustrates the principles for how to create a minimum <iohw.h> implementation. The implementation provides no machine code overhead. You simply get I/O driver source code portability at no extra cost.
This method can be used with many existing embedded C (C++) compilers on the market today.

The I/O register (or ioreg_designator) definition.

     #define DISP_DATA  ((volatile unsigned char  *) 0x0001234)

This definition fulfil the three basic requirements for a valid ioreg_designator definition:

• Provide a symbolic name for the I/O register
• Define the (logical) I/O register size in number of bits
• Define the physical address of the I/O register in the target platform

The iohw function implementation method are here based on macro functions which map to a pointer based syntax for I/O register access used by the given compiler (only functions for I/O write access are shown here)

  #define  iowr(  ioreg_desc, dat )            (((ioreg_desc)[0]) = (dat))
  #define  iowrbuf(  ioreg_desc, index, dat )  (((ioreg_desc)[(index)]) = (dat))

Implementation example 2

When used with IOTester^® tool the I/O driver source codes are the same but access is done via the "intrinsic" iot_xx(..) access functions provided with the IOTester tool.

The I/O register (or ioreg_designator) definition looks nearly the same:

    #define DISP_DATA  (IOT_REG8 |  0x0001234)

Again the definition fulfil the three basic requirements for a valid ioreg_designator definition:

• Provide a symbolic name for the I/O register
• Define the (logical) I/O register size in number of bits
• Define the physical address of the I/O register in the target platform

When used with IOTester the iohw functions just map directly to the corresponding IOTester functions for direct access of target I/O registers from an "embedded" application running on the PC:

#define  iowr( ioreg_desc, dat )             iot_wr( (ioreg_desc), (dat))
#define  iowrbuf(  ioreg_desc, index, dat )  iot_wrbuf( (ioreg_desc), (index), (dat))

A full <iohw.h> implementation is included with IOTester. This header may be used both for PC mode and target mode compilation in cases where the embedded target C compiler does not yet provides an <iohw.h> implementation.

Links to:
IOTester tool description
Example 1: Start using IOTester I/O port pins - "Single chip mode"
Example 2: Enabling the External bus
Example 3: Using Internal Devices and External bus.
Example 4: Using Target Interrupt.
Example 5: Writing portable I/O driver source code with <iohw.h>
Example 6: <iohw.h> implementation methods and definition of I/O registers
Example 7: Swapping source code between PC and target platforms