"add register to register" instruction – one could just use the "add memory to register" instruction. In the case of early models of the PDP-10, which did not have any cache memory, a tight inner loop loaded into the first few words of memory (where the fast registers were addressable if installed) ran much faster than it would have in magnetic core memory. Later models of the DEC PDP-11 series mapped the registers onto addresses in the input/output area, but this was primarily intended to allow remote diagnostics. Confusingly, the 16-bit registers were mapped onto consecutive 8-bit byte addresses. Memory indirect and autoincrement The DEC PDP-8 minicomputer had eight special locations (at addresses 8 through 15). When accessed via memory indirect addressing, these locations would automatically increment prior to use. This made it easy to step through memory in a loop without needing to use the accumulator to increment the address. The Data General Nova minicomputer had 16 special memory locations at addresses 16 through 31. When accessed via memory indirect addressing, 16 through 23 would automatically increment before use, and 24 through 31 would automatically decrement before use. Zero page The Data General Nova, Motorola 6800 family, and MOS Technology 6502 family of processors had very few internal registers. Arithmetic and logical instructions were mostly performed against values in memory as opposed to internal registers. As a result, many instructions required a two-byte (16-bit) location to memory. Given that opcodes on these processors were only one byte (8 bits) in length, memory addresses could make up a significant part of code size. Designers of these processors included a partial remedy known as "zero page" addressing. The initial 256 bytes of memory ($0000 – $00FF; a.k.a., page "0") could be accessed using a one-byte absolute or indexed memory address. This reduced instruction execution time by one clock cycle and instruction length by one byte. By storing often-used data in this region, programs could be made smaller and faster. As a result, the zero page was used similarly to a register file. On many systems, however, this resulted in high utilization of the zero page memory area by the operating system and user programs, which limited its use since free space was limited. Direct page The zero page address mode was enhanced in several late model 8-bit processors, including the WDC 65816, the CSG 65CE02, and the Motorola 6809. The new