/* Borland 16-bit stack-overflow guard and TDKPIN startup entry. */

#include "tdkpin_stack.h"

enum {
    BORLAND_STACK_SAFETY_BYTES = 0x0400,
    BORLAND_STACK_LOWER_LIMIT_OFFSET = 0x000a,
    BORLAND_STACK_LOW_WATER_OFFSET = 0x000e,
};

/*
 * 1020:03cb — check AX bytes plus the Borland 0x400-byte safety reserve.
 * All comparisons are unsigned 16-bit operations against the live SS layout.
 * Runtime Error 202 covers addition overflow, collision with SP, and crossing
 * the absolute lower limit. A successful deeper use updates SS:000e.
 */
uint16_t borland_check_stack(uint16_t requested_bytes)
{
    uint16_t required =
        (uint16_t)(requested_bytes + BORLAND_STACK_SAFETY_BYTES);
    if (required < requested_bytes) {
        borland_runtime_error(202);
    }

    uint16_t stack_pointer = win16_current_stack_pointer();
    if (required >= stack_pointer) {
        borland_runtime_error(202);
    }

    uint16_t remaining = (uint16_t)(stack_pointer - required);
    if (remaining <
        win16_read_stack_u16(BORLAND_STACK_LOWER_LIMIT_OFFSET)) {
        borland_runtime_error(202);
    }
    if (remaining < win16_read_stack_u16(BORLAND_STACK_LOW_WATER_OFFSET)) {
        win16_write_stack_u16(BORLAND_STACK_LOW_WATER_OFFSET, remaining);
    }
    return remaining;
}

/* 1008:298a — startup unit entry: execute the stack guard with AX=0. */
void tdkpin_initialize_stack_guard(void)
{
    borland_check_stack(0);
}