behavior description of SPI flash in pseudo-C

spi-flash-pseudo.c

typedef uint8_t byte;

const int FLASH_SIZE = 0x100000;
const int PAGE_SIZE = 0x100;

byte flash[FLASH_SIZE];
int read_addr = 0;
byte write_buf[PAGE_SIZE];
int write_page = 0;
int write_offset = 0;

void read_start(int a) {
    read_addr = a & 0xffffff;
}
byte read() {
    byte d = flash[read_addr];
    read_addr = (read_addr + 1) % FLASH_SIZE;
    return d;
}

void write_start(int a) {
    memset(buf, 0xff, 0x100);
    write_page = a / PAGE_SIZE * PAGE_SIZE;
    write_offset = a % PAGE_SIZE;
}
void write(byte d) {
    buf[write_offset] = d;
    write_offset = (write_offset + 1) % PAGE_SIZE;
}
void write_end() {
    for (int i = 0 ; i < PAGE_SIZE ; i++) {
        flash[write_page + i] &= buf[i];
    }
}

void erase_4k(int a) {
    memset(flash + (a & 0xfff000), 0, 0x1000);
}
void erase_32k(int a) {
    memset(flash + (a & 0xff8000), 0, 0x8000);
}
void erase_64k(int a) {
    memset(flash + (a & 0xff0000), 0, 0x10000);
}