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On-line Debugging Tool

  (Redirected from Octal Debugging Technique)

On-line Debugging Tool (ODT) was used to describe several debugger programs developed for Digital Equipment Corporation (DEC) hardware. Various operating systems including OS/8, RT-11, RSX-11, and RSTS/E implemented ODT as did the firmware console of all of the LSI-11-family processors including the 11/03, 11/23/24, 11/53, 11/73, and 11/83/84.

The debugger allowed access to memory using octal addresses and data. Within the software systems, the debugger accessed the process's address space. DEC's line of PDP-11 processors did not implement virtual memory, from an operating system perspective, but instead worked off of a fixed address space, which was mapped into a unified view of the program's address space, using an Active Page Register (APR). An APR could map the program's RAM in increments of 4K 16-bit words, to a maximum of 32K. In other words, an APR could map 8 segments of RAM, each limited to 4K. Because of this structure, the maximum RAM an APR was able to map was 32K 16-bit words. In the case of RSTS/E, this usually meant that a Runtime System, or RTS, mapped to the upper portion of the address space and a user program resided in the lower portion of the address space. The RTS provided code to support access to the Operating System, on behalf of the user program; the RTS itself stored any of its non-static data in the address space of the user program, because the RTS was typically read-only. The operating system loaded a single copy of the RTS and this was mapped to any user program that required that RTS. The APR would be set to map the RTS into the upper portion of the program's address space, in 4K increments. So the BASIC Plus RTS (for the Basic+ Programming Language) typically mapped 16K to itself and the user program was mapped, in 4K increments, in the lower 16K. The RT11 RTS occupied 4K so a user program, like the RT11-based Peripheral Interchange Program (PIP), could expand to a maximum of 28K.

ODT could be used to "patch" binary modules, like an RTS, without requiring the re-compilation of the binary's source.

The firmware console implementation accessed physical memory.

ODT is a non-symbolic debugger and implements similar functionality to Advanced Debugger (adb) on Unix systems.


Console ODTEdit

Console ODT[1] replaced the lights and switches console of many of the earlier processors.

Access to console ODT is obtained either from power up (with appropriate power up mode selected), by the execution of a HALT instruction in kernel mode, or by use of the front panel halt switch or button.


@1000/ xxxxxx 112737<LF>
001002 xxxxxx 101<LF>
001004 xxxxxx 177566<LF>
001006 xxxxxx 137<LF>
001010 xxxxxx 1000<CR>
>R7/xxxxxx 1000<CR>

This deposits the program

MOVB 'A', @#177566  ; Move 'A' into console transmit register
JMP @#1000        ; Jump back to start

The deposit to the PC [Program Counter], sets the PC to the start of the program and the deposit to the PSW [Program Status Word] locks out interrupts.

The effect of this will be to write a stream of "A" to the console. As there is no check for transmitter ready, it is highly probable that a large number of garbage characters will be displayed.

RSX-11M-Plus ODTEdit

The RSX-11M-Plus ODT[2] is essentially a superset of all other ODT implementations.

ODT is implemented as code that is linked with a task using the Task Builder /DA switch.


Once any task built with ODT is run ODT is invoked on entry.


The underscore is the standard ODT prompt.

Addresses in the ODT debugger are 16 bit addresses in the mode in which ODT is operating, not the physical addresses used with console ODT.

See alsoEdit


  1. ^ p192-202 PDP-11 Processor Handbook: pdp11/04/24/34a/44/70, Digital, 1981
  2. ^ IAS/RSX-11 ODT Reference Manual, Order Number AA-M507A-TC, Digital, 1982