8-bit PIC Memory Constraints Analysis¶

Overview¶

Analysis of memory limitations in 8-bit PIC microcontrollers and their impact on C++ implementation feasibility.

PIC8 Memory Architecture¶

Harvard Architecture Characteristics¶

• Separate Program and Data Memory: Cannot execute code from data memory
• Limited Data Memory: Typically 32 bytes to 4KB total
• Bank Switching: Data memory divided into banks (128-256 bytes each)
• Special Function Registers: Overlay data memory space
• Stack Limitations: Hardware stack, typically 8-32 levels deep

Memory Map Structure¶

Program Memory (Flash)
├── Reset Vector (0x0000)
├── Interrupt Vector (0x0004/0x0008)
├── User Code Space
└── Configuration Bits

Data Memory (RAM)
├── Bank 0: General Purpose + SFRs
├── Bank 1: General Purpose + SFRs  
├── Bank N: Additional banks
└── Common RAM (accessible from all banks)

Device Memory Specifications¶

PIC10F Family (Ultra Low-End)¶

Analysis: Too constrained for any meaningful C++ usage.

PIC12F Family (Low-End)¶

Analysis: Only the newer 12F devices might support minimal C++.

PIC16F Family (Mid-Range)¶

Analysis: Modern 16F devices with >1KB RAM could support C++.

PIC18F Family (High-End 8-bit)¶

Analysis: All modern 18F devices should support C++ reasonably well.

C++ Memory Overhead Analysis¶

Minimal C++ Runtime Requirements¶

Essential Components¶

// Estimated memory overhead
void* operator new(size_t size);      // ~50 bytes
void operator delete(void* ptr);      // ~30 bytes
void __cxa_pure_virtual();           // ~20 bytes
// Total: ~100 bytes minimum

Class Implementation Overhead¶

class SimpleClass {
    int value;                        // 2 bytes
    void method();                    // No overhead if not virtual
};
// Instance overhead: 2 bytes
// Code overhead: Actual method code only

Virtual Function Overhead¶

class VirtualClass {
    virtual void method();            // +2 bytes for vtable pointer
};
// Per instance: +2 bytes
// Per virtual method: +2 bytes in vtable
// Virtual call: +~10 bytes overhead per call

Memory Usage Examples¶

Simple Class Usage¶

class LED {
    bool state;                       // 1 byte
    uint8_t pin;                      // 1 byte
public:
    void on() { state = true; }       // Inline, no overhead
    void off() { state = false; }     // Inline, no overhead
};

LED led1, led2;                      // 4 bytes total

Template Usage (if supported)¶

template<uint8_t PIN>
class TemplateLED {
    bool state;                       // 1 byte per instance
    // PIN is compile-time constant, no storage
public:
    void toggle() { /* ... */ }       // Code generated per PIN value
};

TemplateLED<2> led1;                  // 1 byte
TemplateLED<3> led2;                  // 1 byte + duplicate code

Feasibility Assessment by Device Class¶

Definitely Feasible (✅)¶

Requirements: >1KB RAM, >8K program memory, modern architecture - PIC18F family (most devices) - PIC16F with Enhanced Mid-Range core and >1KB RAM - Suitable for: Classes, simple inheritance, basic templates

Possibly Feasible (⚠️)¶

Requirements: 256-1KB RAM, >2K program memory - Newer PIC12F devices (12F1822+) - Mid-range PIC16F with sufficient RAM - Suitable for: Simple classes, no inheritance, minimal features

Not Feasible (❌)¶

Requirements: <256 bytes RAM or <2K program memory - PIC10F family (all devices) - Older PIC12F devices - Basic PIC16F devices - Too constrained for meaningful C++ usage

Memory Optimization Strategies¶

Code Size Optimization¶

1. Aggressive Inlining: Eliminate function call overhead
2. Template Specialization: Reduce code duplication
3. Compiler Optimizations: -Os optimization level
4. Feature Subset: Only enable essential C++ features

RAM Usage Optimization¶

1. Stack Management: Careful stack usage monitoring
2. Static Allocation: Avoid dynamic allocation completely
3. Bank Optimization: Efficient use of banked memory
4. Overlay Techniques: Reuse memory for temporary objects

Recommended C++ Subset for 8-bit PICs¶

Allowed Features¶

• ✅ Classes and member functions
• ✅ Constructors and destructors (simple)
• ✅ Function overloading
• ✅ References
• ✅ Inline functions
• ✅ Simple templates (limited instantiation)
• ✅ Operator overloading

Restricted Features¶

• ⚠️ Virtual functions (only if >1KB RAM)
• ⚠️ Inheritance (simple only)
• ⚠️ Multiple inheritance (not recommended)

Prohibited Features¶

• ❌ Dynamic allocation (new/delete)
• ❌ Exceptions
• ❌ RTTI
• ❌ STL
• ❌ Complex templates

Target Device Recommendations¶

Primary Targets (Best C++ Support)¶

1. PIC18F47K42: 64K program, 4KB RAM - Full C++ subset
2. PIC18F4550: 32K program, 2KB RAM - Good C++ support
3. PIC16F18857: 32K program, 2KB RAM - Modern mid-range

Secondary Targets (Limited C++ Support)¶

1. PIC16F877A: 8K program, 368B RAM - Basic C++ classes
2. PIC12F1840: 4K program, 256B RAM - Very limited C++

Testing Priorities¶

1. Start with PIC18F47K42 (best case scenario)
2. Validate on PIC16F18857 (modern mid-range)
3. Test limits with PIC16F877A (minimal viable)
4. Stretch goal: PIC12F1840 (extreme constraint)

Conclusions¶

Feasibility Summary¶

• High-end 8-bit PICs: C++ is definitely feasible with careful design
• Mid-range PICs: Limited C++ subset possible
• Low-end PICs: Not practical for C++ development

Recommended Approach¶

1. Target Modern Devices: Focus on PIC18F and newer PIC16F
2. Subset Implementation: Support only essential C++ features
3. Aggressive Optimization: Prioritize code size and RAM usage
4. Clear Documentation: Define supported subset and limitations

Memory Requirements for Minimal C++ Support¶

• Minimum RAM: 256 bytes (very limited features)
• Recommended RAM: 1KB+ (reasonable C++ subset)
• Optimal RAM: 2KB+ (most C++ features except prohibited ones)
• Program Memory: 4KB minimum, 16KB+ recommended