Register Assignments
The 6000 series has three banks of registers. The data registers (X0-X7)
hold 60-bit arguments and results from the machines more complex computations
such a long integer and floating point arithmetic. All memory references
go through these registers.
-
X0 12-bit word mask (7777 octal)
-
X1
-
X2 PDP-8 Link and Accumulator
-
X3 PDP-8 Memory Address Register (most
times)
-
X4 PDP-8 Memory Data Register (most times)
-
X5 PDP-8 Program Counter
-
X6
-
X7 Instruction Register (during decoding)
The 18-bit index registers are used for address calculation or other counting.
-
B0 Zero (hardware convention)
-
B1 One (frequent software convention)
-
B2 Remaining Instruction Counter
-
B3
-
B4
-
B5
-
B6 Word Block Shift (after FETCH)
-
B7
The 18-bit address registers (mostly) retain the machine addresses of recent
memory operations. Setting some address registers (A1-A5) initiate a memory
read to the corresponding X register. Setting other address registers (A6-A7)
initiate a memory write from their corresponding X registers. A0 (and X0)
are free of this side effect.
-
A0
-
A1 Word Block Address (after FETCH)
-
A2
-
A3
-
A4
-
A5
-
A6
-
A7
PDP-8
Simulator
© 1975, 1998, Ward Cunningham
all rights reserved