Specifications

When the system is running a program, the LOAD ADDR, EXAM, and DE-
POSIT functions are disabled to prevent disrupting the,program. When the
machine is to be halted, the ENABLE/HALT switch is thrown to the halt
position. The machine will halt either at the end of the current instruction,
or at the end of the current bus cycle, depending upon the position of the
S/ INST-S/CYCLE switch.
-OPERATING THE CONTROL SWlTCHES
When the PDP-11 has been halted, it is possible to examine and update bus
locations. To examine a specific location, the operator sets the switches of
the switch register to correspond to the location’s address. The operator then
presses LOAD ADDR, which will transfer the contents of the switch register
into the bus address register. The location of the address to be examined is
then displayed in the address register display. The operator then depresses
EXAM. The data in that location will appear in the data register display.
If the operator then depresses EXAM again, the, bus address register will be
incremented by 2 to the next word address and the new location will be
examined. In the PDP-11, the bus address register will always be pointing to
the data currently displayed in the data register,.The incrementation occurs
when the EXAM switch is depressed, and then the location is examined.
The examine function has been designed so that if LOAD ADDR and then *
EXAM are depressed, the address register will not be incremented. In this
case, the location reflected in the address register display is examined
directly. However, on the second (and successive) depressings of EXAM, the
bus address register is incremented. This will continue for successive de-
pressings as long as another control switch is not depressed.
If.the operator finds an incorrect entry in the data register, he can enter new
data there by putting it in the switch register and raising the DEP key. The
address register will not increment when this.data is deposited. Therefore.
when the operator presses the EXAM key, he can examine the data he just
deposited. However, when he presses EXAM again, the system will increment.
If the operator attempts to examine data from, or deposit data into, a non-
existent memory location, the “time out” feature will cause an error flag. The
data register will then reflect location 4, the trap location, for references to
nonexistent locations. To verify this condition, the operator should try to
deposit some number other than four in that location; if four is still indi-
cated, this would indicate that either nothing is assigned to that location, or
that whatever is assigned to that location is not working properly.
When doing consecutive examines or consecutive deposits, the address will
increment by 2, to successive word locations. However, if the programmer is
examining the fast registers (the “scratch pad” memory), the system only
increments by 1. The reason for this is that once the switch register is set
properly, the programmer can then use the four least significant bitsof the
switch register in examining fast memory registers from the front panel.
To start a PDP-11 program, the programmer loads the starting address of
the program in the switch register, depresses LOAD ADDR, and after ensur-
ing that the ENABLE/HALT switch is in the ENABLE position, depresses
ST;:. The program will ,start to run as soon as the START switch is re-
The Run indicator lamp is driven off the flip-flop that controls the clock.
Normally, when the system is running, not only will this light be on, .but the
89