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16-bit Digital Signal Controllers freescale.com 56800E Coding Guidelines for Small Data Memory and Large Data Memory Model CG56800E Rev. 1

Table of Contents, Rev. 1 Freescale Semiconductor i Preliminary About This Document Chapter 1 Introduction Chapter 2 Coding Guidelines

56800E Coding Guidelines, Rev. 1 ii Freescale Semiconductor Preliminary

List of Examples, Rev. 1 Freescale Semiconductor iii Preliminary

56800E Coding Guidelines, Rev. 1 iv Freescale Semiconductor Preliminary

Preface, Rev. 1 Freescale Semiconductor v Preliminary About This Document This manual describes some coding guidelines in order to generate valide code for both a Small Data Memory model (SDM) and a Large Data Memory model (LDM) target for a 56800E application. Audience This document targets software developers who have implemented software applications with 56800 devices and are converting to the 56800E family of devices.

56800E Coding Guidelines, Rev. 1 vi Freescale Semiconductor Preliminary Conventions This document uses the following notational conventions: Typeface, Symbol or Term Meaning Examples Courier Monospaced Type Code examples //Process command for line flash Italic Directory names, project names, calls, functions, statements, procedures, routines, arguments, file names, applications, variables, directives, code snippets in text ...and contains these core directories: applications contains applications software... Bold Reference sources, paths, emphasis ...refer to the Targeting 56F80x Platform manual.... Number Any number is considered a positive value, unless pre- ceded by a minus symbol to signify a negative value -10 DES-1 ALL CAPITAL LETTERS # defines/ defined constants # define INCLUDE_STACK_CHECK Quotation marks, “...” ...if unsuccessful for any reason, it will return “NULL”...

Preface, Rev. 1 Freescale Semiconductor vii Preliminary Definitions, Acronyms, and Abbreviations The following list defines the acronyms and abbreviations used in this document. As this template develops, this list will be generated from the document. As we develop more group resources, these acronyms will be easily defined from a common acronym dictionary. Please note that while the acronyms are in solid caps, terms in the definition should be initial capped ONLY IF they are trademarked names or proper nouns. DSC Digital Signal Controller LDM Large Data Memory SDM Small Data Memory References The following sources were referenced to produce this book: 1. DSP56800 to DSP56800E Porting Guide, DSP56800ERG 2. DSP56800 Family Manual, DSP56800FM 3. DSP56800E Reference Manual, DSP56800ERM 4. Inside CodeWarrior: Core Tools

56800E Coding Guidelines, Rev. 1 viii Freescale Semiconductor Preliminary

Introduction, Rev. 1 Freescale Semiconductor 1-1 Preliminary Chapter 1 Introduction

1.1 Switching Between Models

A Small Data Memory model (SDM) and a Large Data Memory model (LDM) are available for the 56800E family of controllers. The SDM has a maximum address size of 16 bits for both data and program addresses. This allows for smaller, compact code size, since an address can be stored in one 16-bit word. The LDM has a maximum address size of 19 bits for program addresses and 21 bits for data addresses. This allows an application to use more memory, but it requires two 16-bit words of memory to store each address. In order to switch between the different models during development, specific coding practices should be followed to make this as transparent as possible. The remainder of this document describes these practices.

56800E Coding Guidelines, Rev. 1 1-2 Freescale Semiconductor Preliminary

LoadRx, StoreRx, and TestRx Macros Coding Guidelines, Rev. 1 Freescale Semiconductor 2-1 Preliminary Chapter 2 Coding Guidelines This section explains how the user’s source code should be developed in order to generate code for both the SDM and LDM.

2.1 Type Casting Pointers

When developing software for the SDM, do not type cast pointers to be 16 bits (i.e., unsigned int). This will work for the SDM, but if you switch to the LDM, it may not work, because your pointer size may be greater than 16 bits.

2.2 Conversion of 56800 Instructions to 56800E Instructions

When the 56800E was developed, one requirement was the ability to use all of the 56800 instructions to easily upgrade from the 56800 to the 56800E. This approach works fine for the SDM; however, since the 56800 instructions are for a 16-bit controller with 16-bit pointers, this approach is no longer valid when using the LDM. When using the LDM, the “allow legacy instructions” Language Settings: M56800E Assembler option must be turned off and the user must convert all 56800 instructions to 56800E instructions. Please see the DSP56800 to DSP56800E Porting Guide to learn more.

2.3 LoadRx, StoreRx, and TestRx Macros

When loading, storing, or testing pointers in Assembly, you must specify if it is a word or a long. For the SDM, pointers are a word and for the LDM, pointers are a long. Depending on the model, defined macros in portasm.h ease switching from SDM to LDM and vice versa. These assembly macros (LoadRx, StoreRx, and TestRx) are defined based on which model is used, so they load, store, and test the correct size of the pointer.

56800E Coding Guidelines, Rev. 1 2-2 Freescale Semiconductor Preliminary Example 2 -1. Use Macros to Convert 56800 or 56800E Assembly Code

56800 Assembly code:

move X:(R2+Offset_pCircBuffer),R0 ; where pCircBuffer contains a pointer move R0,X:(R2+Offset_pCircBuffer) tstw X:(R2+Offset_pCircBuffer) or 56800E Assembly code (SDM): moveu.w X:(R2+Offset_pCircBuffer),R0 ; where pCircBuffer contains a pointer move.w R0,X:(R2+Offset_pCircBuffer) tst.w X:(R2+Offset_pCircBuffer) or 56800E Assembly code (LDM): move.l X:(R2+Offset_pCircBuffer),R0 ; where pCircBuffer contains a pointer move.l R0,X:(R2+Offset_pCircBuffer) tst.l X:(R2+Offset_pCircBuffer) can be converted to: LoadRx X:(R2+Offset_pCircBuffer),R0 ; where pCircBuffer contains a pointer StoreRx R0,X:(R2+Offset_pCircBuffer) TestRx X:(R2+Offset_pCircBuffer)

2.4 Push and Pop Instructions

The 56800 has instructions for push and pop that save and restore 16-bit address registers. The push and pop instructions should not be used for the 56800E, since these instructions temporarily even the stack. This could cause alignment access problems for long data types, since long data types must be on odd addresses. If 56800 Assembly code is being ported to the 56800E and it contains push or pop instructions, this code should be rewritten so it does not use these instructions.

Coding Guidelines, Rev. 1 Freescale Semiconductor 2-3 Preliminary Example 2 -2. Converting 56800 Assembly Code Using Push and Pop #1 ... pop R2 can be converted to: adda #2,sp move.l R2,x:(sp) ... move.l x:(sp)-,R2 Example 2 -3. Converting 56800 Assembly Code using Push and Pop #2 ... pop Y1 pop Y0 can be converted to: adda #2,sp move.w Y0,x:(sp) move.w Y1,x:(sp-1) ... move.w x:(sp-1),Y1 move.w x:(sp),Y0 suba #2,sp

56800E Coding Guidelines, Rev. 1 2-4 Freescale Semiconductor Preliminary

2.5 Structure Offsets

If your assembly code accesses elements of a structure defined in C, offsets to the elements of the structure must be defined in Assembly. In portasm.h, a PTR_SIZE macro has been created that defines the size of a pointer (1 for SDM and 2 for LDM). To seamlessly switch from SDM to LDM and vice versa, assembly code that defines offsets to these elements should be relative to the previous element and PTR_SIZE should be used when specifying the size of a pointer. Also, structure elements that are longs should be the first elements in the structure, because CodeWarrior requires that long values start on an odd address. Structure elements that are pointers should follow because they will be longs for LDM. By having all of the longs and pointers first, the start of the structure will be on an odd address and there will be no gaps in the structure. If longs or pointers were placed in the middle of the structure, then gaps would be inserted to guarantee an odd address. Because Assembly code is written to assume no gaps, this would cause a problem. Example 2 -4. Convert C structure If C structure is defined as follows: struct C_Structure { bool bStatus; int * pCircBuffer; int Size; UWord32 LongWord; then you can change the C structure to the following: struct C_Structure { UWord32 LongWord; int * pCircBuffer; bool bStatus; int Size;

Coding Guidelines, Rev. 1 Freescale Semiconductor 2-5 Preliminary Example 2 -5. Convert Assembly Code Structure If Assembly code defines these offsets to each element as follows: Offset_bStatus equ 0 Offset_pCircBuffer equ 1 Offset_size equ 2 Offset_LongWordequ 3 then you can change the Assembly code to the following: Offset_LongWordequ 0 Offset_pCircBuffer equ Offset_LongWord+2 Offset_bStatus equ Offset_pCircBuffer+PTR_SIZE Offset_size equ Offset_bStatus+1

56800E Coding Guidelines, Rev. 1 2-6 Freescale Semiconductor Preliminary

Index, Rev. 1 Freescale Semiconductor Index-i Preliminary D DSP vii Digital Signal Processor vii L LDM vii Large Data Memory Model vii LoadRx 1 long 1 longs 4 M M56800E Assembler 1 P pointers 4 pop 2 portasm.h 1, 4 PTR_SIZE macro 4 push 2 S SDM vii Small Data Memory Model vii StoreRx 1 T TestRx 1 W word 1

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