#include <stdio.h>
#include <stdint.h>

int main(void) {
    printf("VAX-like 32-bit address space simulation (512-byte pages)\n");
    printf("VPN = VA >> 9 (23 bits); segment from top 2 bits of VPN\n\n");
    const uint32_t examples[7] = {
        0x00000000u,
        0x00001000u,
        0x3FFFFFFFu,
        0x40000000u,
        0x7FFFFFFFu,
        0x80000000u,
        0xC0000000u
    };
    const char *names[7] = {
        "null page",
        "user code",
        "end of P0 heap",
        "start of P1 stack",
        "end of P1",
        "kernel start",
        "unused"
    };
    for (int i = 0; i < 7; i++) {
        uint32_t va = examples[i];
        uint32_t offset = va & 0x1FFu;
        uint32_t vpn = va >> 9;
        uint32_t seg = (vpn >> 21) & 3u;
        const char *segname;
        switch (seg) {
            case 0: segname = "P0 (process)"; break;
            case 1: segname = "P1 (process)"; break;
            case 2: segname = "S (system)"; break;
            default: segname = "unused"; break;
        }
        printf("VA=0x%08X  VPN=%7u  off=%3u  seg=%s  [%s]%s\n",
               va, vpn, offset, segname, names[i],
               (va < 512 ? " INVALID" : ""));
    }
    printf("\nNote: page 0 is marked invalid to catch null-pointer bugs.\n");
    printf("Kernel (S) is mapped in every process for easy pointer passing.\n");
    return 0;
}