IBM System/360 Operating Guide

CPU Check Indicators
In the central processing unit are several check
circuits. Whenever a check is detected, a check
latch is set. and a bit is turned on in the machine-
check (MC) register, as follows:
MC Register
Position
Indication
0
A-REG
1
B-REG
2
STaR ADR
3
CTRL REG
4
ROS SALS (Read-Only
Storage Sense Amplifier
Latches)
5
ROS ADR
6
STaR DATA
7
ALU
Except for the ALU check, the turning on of
any of these lights is an indication of detected bad
parity in the associated register. In the ALU, a
duplicate check is made because the ALU does not
use the parity bit (input is complemented and the
operation is performed in both uncomplemented and
complemented form, and the answers are compared).
When the CPU stops during normal program proces-
sing with anyone or more of these indicators on,
call the Customer Engineer.
Registers
The following registers have full-time indicators
on the 2030 console (left center and upper left
portions of the console, Figure 3).
The ALU Output is not a register but is listed
here because of its relationship with the actual
registers.
B- and A-Register Displays
These indicators provide full-time display of the
contents of the B- and A-registers.
During manual operations, the display from the
A-register depends on the settings of console
switches (see the Display Key section).
When the system is in the Wait state (instruction
processing stopped and the program is waiting for
an interruption) or when in the process-stop con-
dition (such as when the Stop key is pressed), the
instruction counter (IC) is displayed in the B-register
(I-register contents) and the A-register (J-register
contents).
16
Main-Storage Data Register (MSDR) Display
For CPU or multiplexor-channel operations, these
indicators dis play information being transferred to
or from either main or auxiliary storage. The
MSDR indicators, therefore, provide full-time
display of the contents of the R-register.
For manual operations, the MSDR indicators
display the data byte being stored, displayed, etc.,
depending upon the setting of 2030 console switches.
ALU Output Display
These indicators provide full-time dis play of the
output of the arithmetic-logic unit (ALU).
MAIN STaR Indicator
When on, this light (located to the right of the MSAR
lamps) indicates that a main-storage address is
being displayed by the MSAR lights.
AUX STaR Indicator
When on, this light (located to the right of the
MSAR lamps) indicates that an auxiliary-storage
address is being displayed by the MSAR lights.
Main Storage Address Register (MSAR) Display
When the MAIN STaR indicator is on during normal
processing operations these lights display the
address of the main-storage location being accessed.
During manual operations, the information
dis played in the MSAR lights may apply to any of
several internal system registers, depending upon
the setting of the Display Storage Selection dial
(switch E) and/or rotary switches A, B, C, and D.
When
the AUX STaR indicator is on, the high-
order hexadecimal digit (four high-order bits) of
the MSAR specify which part of auxiliary storage
is addressed (that is, local storage or one of the
MPX storages).
Note that when the system is in Wait state or in
the process-stop condition, the address of the next
instruction to be executed (instruction counter--IC)
is displayed in the B- and A-registers. The opera-
tion code is not displayed.

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English