The zero page or base page is the series of memory addresses at the very beginning of a computer's address space; that is, the page whose starting address is zero. The size of a "page" depends on the context, and the significance of zero-page memory versus higher addressed memory is highly dependent on machine architecture. For example, the Motorola 6800 and MOS Technology 6502 processor families treat the first 256 bytes of memory specially, whereas many other processors do not. The actual size of the zero page in bytes is determined by the microprocessor design and in older designs, is often equal to the largest value that can be referenced by the processor's indexing registers. For example, the aforementioned 8-bit processors have 8-bit index registers and a page size of 256 bytes. Therefore, their zero page extends from address 0 to address 255. In early computers, including the PDP-8, the zero page had a special fast addressing mode, which facilitated its use for temporary storage of data and compensated for the relative shortage of CPU registers. The PDP-8 had only one register, so zero page addressing was essential. Unlike more modern hardware, in the 1970s computerRAM was as fast as or faster than the CPU. Thus it made sense to have few registers and use the main memory as an extended pool of extra registers. Since each memory location within the zero page of a 16-bitaddress bus computer may be addressed by a single byte, it was faster, in 8-bit data bus machines, to access such a location rather than a non-zero page location. For example, the MOS Technology 6502 has only one general purpose register. As a result, it used the zero page extensively. Many instructions are coded differently for zero page and non-zero page addresses: LDA $00 ; zero page LDA $0000 ; non-zero page
The above two instructions both accomplish the same thing: they load the value of memory location $00 into the.A register. However, the first instruction is only two bytes long and requires three clock cycles to complete. The second instruction is three bytes in length and requires four clock cycles to execute. This difference in execution time could become significant in repetitive code. Zero page addressing now has mostly historical significance, since the developments in integrated circuittechnology have made adding more registers to a CPU less expensive and CPU operations much faster than RAM accesses.
Null pointers
Contrary to the zero page's original preferential use, some modern operating systems such as FreeBSD, Linux and Microsoft Windows actually make the zero page inaccessible to trap uses of null pointers. Such pointer values may legitimately indicate uninitialized values or sentinel nodes, but they do not point to valid objects. Buggy code may try to access an object via a null pointer, and this can be trapped at the operating system level as a memory access violation.
Interrupt vectors
Some computer architectures still reserve the beginning of address space for other purposes, though; for instance, Intel x86 systems reserve the first 256 double-words of address space for the interrupt vector table if they run inreal mode. A similar technique of using the zero page for hardware related vectors was employed in the ARM architecture. In badly written programs this could lead to "ofla" behaviour, where a program tries to read information from an unintended memory area, and treats executable code as data or vice versa. This is especially problematic if the zero page area is used to store system jump vectors and the firmware is tricked into overwriting them.
In 8-bit CP/M, the zero page is used for communication between the running program and the operating system.
Page addressing
In some processor architectures, like that of the Intel 4004 4-bit processor, memory was divided into pages and special precautions had to be taken when the control flow crossed page boundaries, as some machine instructions exhibited different behaviour if located in the last few instructions of a page, so that only few instructions were recommended to jump between pages.
