Ciprico Rimfire 3880 manual Need for Swapping, Details of Usage Byte and Word Swapping

Chapter 8 Details of Usage

Byte and Word Swapping Page 8 - 3

Byte and Word Swapping

The RF3880 adapter is flexible enough to allow you to request Byte Swapping, Word Swapping, or both, during transfers of command structures.

This section will first give you some examples to explain why the swapping feature is offered to you, then provide a perspective on implementation with the RF3880 for both Command Structures used by the RF3880 (which the adapter can swap) and data (which needs special software if swapping is necessary).

Need for Swapping

The memory architecture of your system depends on the type of processor you are using. There are several major manufacturers that each use different arrangements of bytes, words, and double-words in memory. The two most common formats are: Intel and Motorola. Their differences are illustrated in the example below:

Example of Memory Differences

If you were to define the following variables in a program:

char byte[] = “test”;

int word[2] = {0x0102, 0x0304}; double dword = 0xAABBCCDD;

They would appear this way in memory (beginning at location n) for each respective format:

variable	Byte # of memory		Intel Order				Motorola Order
byte	n	t	e	s	t	t	e	s	t
word	n + 4	02	01	04	03	01	02	03	04
dword	n + 8	DD	CC	BB	AA	AA	BB	CC	DD

Table 108: Variations in Intel and Motorola Memory

Essentially the two processors store a string of bytes in the same order. However, the bytes that make up word and double-word information are stored in an order opposite to each other:

The RF3880 adapter must correctly access the command structures for either type of processor.

Processor Type	Byte of word or	Order in memory
	double-word
Motorola	LSB*	stored in highest memory address
	MSB*	stored in lowest memory address
Intel	LSB	stored in lowest memory address
	MSB	stored in highest memory address

Table 109: Explanation of Memory Storage Differences

Throughout this manual, the Parameter and Status Blocks that make up the command structures are represented double-word wide, in Motorola ordering.

If you have an Intel processor, you will need to translate these displays to your format. The following example will help explain this.

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