View source for PDP-11 Bootstrap Loader

The '''PDP-11 Bootstrap Loader''' is a short [[program]] designed to be loaded into [[main memory]] on a [[PDP-11]] via the [[front panel]]; it is then run to load an initial program into the machine. I.e it is a way of [[bootstrap]]ping a totally empty machine, one with no [[Read-only memory|ROM]].

It is designed to be absolutely as short as possible (since it is manually toggled in via the [[switch register]]), and thus has limitations on how long a program can be loaded using it, and where in memory that program can be placed. The usual program to load with it is the [[PDP-11 Absolute Loader]], which relaxes both of those limitations.

The original bootstrap loader is [[self-modifying code]], since this allows a very short program. The program is not directly self-modifying; rather, the format of its input is such that it modifies the bootstrap loader as the input is read. (Most bootstrap loader  input files restore the bootstrap loader to its original condition, allowing it to be re-used.)

Several PDP-11 bootstrap ROMs, e.g. the [[BM792 ROM|M792-YA]], M792-YK and [[BM873 ROM]] are prepared to read programs in bootstrap loader format. The code in them is obviously not self-modifying, but reacts to the bootstrap loader format to produce the same effects.

==Structure==

The bootstrap loader contains two locations which are modified by reading the input: the first is a [[pointer]] to the [[buffer]] in which to place data being read in; the second is a [[conditional branch]] back to the beginning of the data input [[loop]].

The program starts with the pointer set to itself, so the blank leader at the start of the data for the bootstrap loader (originally [[paper tape]]s) starts with a special leader code which causes this to be left un-changed.

The buffer pointer is then modified to place it immediately before the bootrap loader in memory, so at the end of the load the newly loaded code then progressively overlaps the first several [[instruction]]s of the bootstrap loader itself, finally patching the buffer pointer to point at the branch, which is then modified to jump to the newly loaded code.

==Bootstrap Loader format==

The format of boostrap loader input is as follow:

* Leader (all bytes 0351)
* Load offset (1 byte)
* Program to be loaded (up to 0344 bytes)
* Copy of boostrap loader (first 7 bytes)
* Branch offset (1 byte)

==Further reading==

The Bootstrap Loader is documented in some detail in the "paper tape software programming handbook" (DEC-11-GGPB-D), pp. 6-1 to 6-8.