Editing Machine code (section)

===IBM 709x===
The [[IBM 700/7000 series#Later scientific architecture (704/709/7090/7094)|IBM 704, 709, 704x and 709x]] store one instruction in each instruction word; IBM numbers the bit from the left as S, 1, ..., 35. Most instructions have one of two formats:

;Generic
:S,1-11
:12-13 Flag, ignored in some instructions
:14-17 unused
:18-20 Tag
:21-35 Y

;Index register control, other than TSX
:S,1-2 Opcode
:3-17 Decrement
:18-20 Tag
:21-35 Y

For all but the [[IBM 7094]] and 7094 II, there are three index registers designated A, B and C; indexing with multiple 1 bits in the tag subtracts the [[logical or]] of the selected index registers and loading with multiple 1 bits in the tag loads all of the selected index registers. The 7094 and 7094 II have seven index registers, but when they are powered on they are in ''multiple tag mode'', in which they use only the three of the index registers in a fashion compatible with earlier machines, and require a Leave Multiple Tag Mode ('''LMTM''') instruction in order to access the other four index registers.

The effective address is normally Y-C(T), where C(T) is either 0 for a tag of 0, the logical or of the selected index registers in multiple tag mode or the selected index register if not in multiple tag mode. However, the effective address for index register control instructions is just Y.

A flag with both bits 1 selects indirect addressing; the indirect address word has both a tag and a Y field.

In addition to ''transfer'' (branch) instructions, these machines have skip instruction that conditionally skip one or two words, e.g., Compare Accumulator with Storage (CAS) does a three way compare and conditionally skips to NSI, NSI+1 or NSI+2, depending on the result.