Storage Registers; S Register; Z Register; Read/Write Characteristics - Control Data 3100 Reference Manual

STORAGE REGISTERS
S REGISTER
The 13-bit S register contains the address of the word being currently processed. Bit
12 specifies field 0 or field 1 in the memory stack. Bits 00-11 specify the co-ordinates
of the word.
Z REGISTER
The 28-bit Z register is the storage restoration and modification register. All data that
is transferred to or from the storage module passes through Z.
READ/WRITE CHARACTERISTICS
During a normal memory cycle, all bits of a word referenced by (S)* are read out of
core storage in parallel, loaded into Z, used for some purpose, then written back into
storage, intact. Five modes exist in the 3100 Computer for storage modification. In all
cases, Z is initially in the cleared state.
The Z register is only cleared at the beginning of each memory cycle (except in the
case of a Master Clear). If the program stops as the result of a parity error, the operator
can examine
(Z)
on the Storage Module Control Panel, Figure 2-1.
SINGLE-CHARACTER MODE
Anyone character may be ignored during the Read cycle. New data is then loaded
into the corresponding character position of
Z
and the whole
(Z)
is stored.
DOUBLE-CHARACTER MODE
The upper, middle, or lower half of a word is ignored during the Read cycle. New data
is loaded into the unfilled half of
Z
and the whole
(Z)
is stored.
TRIPLE-CHARACTER MODE
Either of the two possible triple-character groups may be ignored during the Read cycle.
New data is then loaded into the corresponding character positions of Z and the whole
(2)
is stored.
FULL-WORD MODE
The whole word is ignored during the Read cycle. A new word is entered into Z and
(Z)
is stored.
ADDRESS MODE
The Jower 15 or 17 bits of a word may be ignored during the Read cycle. A new word
or character address is then loaded into Z, and the whole (Z) is stored.
*The parentheses are an accepted method for expressing the words "the content(s) of" (in this case, "the
contents of S").
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