Editing PDP-10 (section)

====Input/output instructions====
The PDP-10 does not use [[Memory-mapped I/O and port-mapped I/O|memory-mapped devices]], in contrast to the [[PDP-11]] and later DEC machines. A separate set of instructions is used to move data to and from devices defined by a device number in the instruction. Bits 3 to 9 contain the device number, with the 7 bits allowing a total of 128 devices. Instructions allow for the movement of data to and from devices in word-at-a-time (DATAO and DATAI) or block-at-a-time (BLKO, BLKI).{{sfn|Programming|1970|pp=87-88}}

In block mode, the value pointed to by E is a word in memory that is split in two, the right 18 bits indicate a starting address in memory where the data is located (or written into) and the left 18 bits are a counter. The block instructions increment both values every time they are called, thereby increasing the counter as well as moving to the next location in memory. It then performs a DATAO or DATAI. Finally, it checks the counter side of the value at E, if it is non-zero, it skips the next instruction. If it is zero, it performs the next instruction, normally a JUMP back to the top of the loop.{{sfn|Programming|1970|p=88}} The BLK instructions are effectively small programs that loop over a DATA and increment instructions, but by having this implemented in the processor itself, it avoids the need to repeatedly read the series of instructions from main memory and thus performs the loop much more rapidly.{{sfn|Programming|1970|p=89}}

The final set of I/O instructions are used to write and read condition codes on the device, CONO and CONI.{{sfn|Programming|1970|p=86}} Additionally, CONSZ will perform a CONI, bitmask the retrieved data against the value in E, and then skip the next instruction if it is zero, used in a fashion similar to the BLK commands. Only the right 18 bits are tested in CONSZ.{{sfn|Programming|1970|p=87}}