Editing GE-600 series (section)

===Addressing modes===
The 600 series has an elaborate set of addressing modes, many of which use [[indirect word]]s, some of which are auto-incrementing or auto-decrementing.  Multiple levels of indirect addressing are supported.  Indirect addresses have the same format as instructions, and the address modification indicated by the tag field of the indirect address are performed at each level.<ref name="ge-635-manual" />{{rp|page=II{{hyp}}23}}

The tag field of the instruction consist of a 2-bit ''tag modifier'' (tm) and a 4-bit ''tag designator'' (td).<ref name="ge-635-manual" />{{rp|page=II{{hyp}}24}} The tag modifier indicates the type of modification to be performed on the instruction address:<ref name="ge-635-manual" />{{rp|page=II{{hyp}}25}}
* '''Register (R)''': Add the address field (Y) to the contents of the register indicated by the tag designator. 
* '''Register then indirect (RI)''': Perform the address modification as in Register modification, use the word at the effective address as an indirect address of the operand.
* '''Indirect then register (IR)''': Obtain the indirect word from the address specified by Y, and perform the modification requested by the tag field of the indirect word. This may result in multiple levels of indirection.  Perform the address modification specified by the instruction on the last indirect word encountered.
* '''Indirect then tally (IT)''': Obtain the indirect word from the address specified by Y, then use the address in the indirect word as the effective address. Bits 30-35 of the indirect word contained a ''tally'' field which could be used for addressing characters within a word.

For modification types R, RI, and IR the tag designator contains a register to be used for indexing (X0-X7,AU,AL,QU,QL,IC). Other TD values indicate that Y should be used as an immediate operand.  Direct addressing is a special case where Y is used as the operand address with no modification.<ref name="ge-635-manual" />{{rp|page=II{{hyp}}26}}

For modification type IT, the indirect word contains an 18-bit address, a 12-bit tally, and a 6-bit tag. The tag designator indicates the operation to be performed, some of which increment the address and decrement the tally of the indirect word or decrement the address and increment the tally of the indirect word. The '''Character from Indirect''' and '''Sequence Character'''  operations can be used to address 6-bit and 9-bit [[byte]]s; this supports extracting specific bytes, and incrementing the byte pointer, but not random access to bytes.<ref name="ge-635-manual" />{{rp|page=II{{hyp}}26-II{{hyp}}33a}}<ref>cf. [[Byte addressing#Hybrid systems]]</ref>