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String Pool Memory Allocation Changes (2025-11-23)

Summary

Changed from malloc-based heap allocation to direct memory allocation from BSS_tail.

OLD SYSTEM (REMOVED)

  • String pool allocated via malloc() from z88dk heap
  • Compiled with -DAMALLOC flag to enable heap
  • mb25_init(pool_size) called malloc internally
  • mb25_cleanup() called free()
  • Fixed pool size (default 2048 bytes, configurable)
  • Max usable: ~39KB due to heap management overhead
  • Extra ~1KB code for heap allocator

NEW SYSTEM

  • String pool uses memory directly from __BSS_tail to SP - stack_reserve
  • No -DAMALLOC flag (no heap linked in)
  • mb25_init(pool_address, pool_size) receives pre-calculated values
  • No malloc/free needed
  • Dynamic pool size based on available memory
  • Max usable: ~56KB on 64K CP/M system
  • Smaller binary (no heap code)

Memory Layout

0x0100 ┌─────────────────────┐ TPA Start
       │  Program Code       │
       │  Static Variables   │
       │  FCBs (file I/O)    │ ← File buffers in BSS, not heap!
       ├─────────────────────┤ __BSS_tail
       │                     │
       │  STRING POOL        │ ← All free memory for strings
       │  (dynamic size)     │
       │                     │
       ├─────────────────────┤ SP - 1024
       │  Stack reserve      │
       └─────────────────────┘ SP

Key Findings

  1. File I/O doesn't use heap - FCBs are static arrays in BSS
  2. z88dk heap adds overhead - both code size and memory management
  3. Direct allocation is simpler - just use the memory after BSS

Files Changed

  • runtime/strings/mb25_string.c - New init signature, no malloc/free
  • runtime/strings/mb25_string.h - Updated function signature
  • src/codegen_backend.py - Remove -DAMALLOC, generate pool calculation code

Codegen Changes

Generated startup code now includes: 

/* Get BSS end and SP for pool calculation */
extern unsigned char __BSS_tail;
static uint16_t _get_sp(void) __naked {
    __asm__("ld hl,0\n add hl,sp\n ret");
}

int main(void) {
    uint16_t pool_start = (uint16_t)&__BSS_tail;
    uint16_t pool_size = _get_sp() - 1024 - pool_start;
    mb25_init((uint8_t*)pool_start, pool_size);
    ...
}

Additional: Shell Sort Replaces qsort

The garbage collector now uses inline shell sort instead of stdlib's qsort: - Avoids linking qsort from stdlib (no stdlib.h needed) - Shell sort is efficient for small arrays (typically <100 strings) - Same O(n log n) complexity as qsort - Implemented in sort_by_data_ptr() and sort_by_str_id()