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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 AppendixUser Stack
The C16x architecture supports stack-relative addressing using the MOV [Ri+#displacement] and MOV [Ri-] instructions.
The Keil C166 Compiler uses the R0 register to maintain a user stack for parameter passing and for automatic (local) variables. The stack grows down from the top of memory. The size of the user-stack area and the user-stack pointer are defined and initialized in the startup code.
When passing arguments to a called function, the compiler stored them on the user stack in reverse order while decrementing the user stack pointer (R0) before each argument is "pushed" onto the stack. When a function with local variables is entered, it decrements the user stack pointer to make space for those local variables. For example:
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Note
- The C166 Compiler optimizes some arguments and local variables into registers, which improves program performance and reduces the amount of stack space required. Optimizer level 0 disables this optimization and forces all arguments and local variables onto the stack. Refer to the OPTIMIZE directive for more information.
- Single-byte variables (char and unsigned char) are passed as words on the user stack. The C166 Compiler inserts memory gaps to ensure the objects on the user stack are word-aligned. Word alignment of 16-bit, 32-bit, and 64-bit objects is required by the C16x architecture. Memory gaps are minimized as much as possible.
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