PDP11GUI Register window
Devices in a PDP-11 are represented by one or more memory addresses in the UNIBUS address space. This is usually called “memory mapped I/O”. In the machine description file several devices can be defined as groups of registers. Register name and additional information is assigned to each memory address.
In the “Memory” menu of PDP11GUI all defined register groups are listed and can be displayed simultaneously.
Two examples are given:
Adresses are even 18 bit or 22 bit physical addresses. You can edit the 16-bit values, and write them back to the target machine by pressing a “Deposit” button . If values have been edited, they are marked yellow until a “Deposit” or a “Examine” is performed (I changed CPU register R0 in the picture above). “Deposit changed” writes back only “yellow” cells, this is an optimization over “Deposit all”
“Examine cur.” updates just the one cell you marked with a mouse click.
If a memory location is changed by use of another tool window, this windows content is updated automatically
The value “?” means that a register’s value is unknown. This is true after startup until the first “Examine” button is pressed.