Control Data Cyber 170 State Hardware Reference Manual page 145

PP Arithmetic Instructions
Add (dl
ADD d
This instruction adds the content at location d (treated as a 12-bit positive
quantity) to the
A
register.
This instruction adds to the content of the
A
register a 12-bit operand
(treated as a positive quantity) obtained by indirect addressing. Location d
is read from PPM, and the word read is used as the operand address.
51dm Add (m
+
(dl) ADM
m,d
This instruction adds the 12-bit operand (treated as a positive quantity) read
by indexed direct addressing to the A register.
In indexed direct addressing, the quantity m, which is read from PPM location P
plus 1, serves as the base operand address to which the content of d is added.
If d equals 0, the operand address is m, but if d is not equal to 0, m plus the
content in d is the operand address. Therefore, location d may be used as an
index quantity to modify operand addresses.