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C166 User's Guide
C166 Introduction Compiling Programs Language Extensions Preprocessor 
Advanced Programming Sections, Classes, and Groups 
Special Sections Locating Sections Customization Files Startup Code STARTUP.A66 START167.A66 START_V2.A66 START_V3.A66 Basic I/O Memory Allocation TRAPS.C Optimizer Optimizer Options General Optimizations 80C166-Specific Optimizations Global Register Coloring DPP Registers Stacks System Stack User Stack Stack Sizes System Stack Size User Stack Size XBUS Memory and Peripherals On-chip XRAM XBUS Peripherals Interfacing C to Assembler Function Parameters Function Return Values Using the SRC Directive Register Usage Example Routines TINY Model SMALL Model COMPACT Model MEDIUM Model LARGE Model Data Storage Formats Bit Variables 1-byte Scalars 2-byte Scalars 4-byte Scalars float Scalars float Errors double Scalars double Errors near Pointers far Pointers huge Pointers xhuge Pointers Absolute Memory Access Absolute Access Macros Linker Location Controls Debugging Error Messages Library Reference AppendixRegister Usage
Assembler functions may destroy the contents of R1-R12, PSW, MDL, MDH, MDC, and DPP0. When invoking a C function from assembly, assume that these registers are destroyed.
When using the MAC directive the compiler uses in addition the MAC registers MSW, MAL, MAH, MRW, and IDX0. When code is generated with the MAC directive, these registers are used by C functions (in addition to the register listed above). When invoking a C function form assembly you need therefore to assume that these registers are destroyed too.
The following registers have special meaning and must be preserved by the assembler subroutine.
| Register | Description |
|---|---|
| R0 | R0 is the user stack pointer. At the end of the assembler subroutine the value of R0 must be the same as at the start of the routine. |
| R13 | This register may be used but its contents must be saved (on entry) and restored (before returning). |
| R14 | This register may be used but its contents must be saved (on entry) and restored (before returning). |
| R15 | This register may be used but its contents must be saved (on entry) and restored (before returning). |
| DPP1 | This DPP register may not be modified by assembler subroutines. It contains the memory class page and may be used in an interrupt service routine that interrupts the assembler subroutine. |
| DPP2 | This DPP register may not be modified by assembler subroutines. It contains the memory class page and may be used in an interrupt service routine that interrupts the assembler subroutine. |
| DPP3 | If DPP3 is modified in the assembler subroutine, it must be reset to 3 (SYSTEM PAGE) before returning. |
Note
- DPP0 is used only when the MOD167 C166 Compiler directive is not specified.
- If your assembler programs alter DPP0, the DPPUSE L166 Linker directive may not be used to assign DPP0 to the near memory area.
- If your assembler programs alter DPP0 or DPP3, the NODPPSAVE C166 Compiler directive may not be used to bypass saving and restoring DPP0 and DPP3 in interrupts.
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