PDP-11 Absolute Loader - Revision history

Jdl: /* Further reading */ Reference to assembly listing for Absolute Loader

2025-03-07T19:43:41Z

‎Further reading: Reference to assembly listing for Absolute Loader

Jnc: Better cat

2023-11-16T17:51:17Z

Better cat

Jnc: typo

2022-04-06T02:37:27Z

typo

Jnc: +cat, links

2022-04-05T20:59:32Z

+cat, links

Jnc: ..LDA used extensively

2019-08-13T13:40:48Z

..LDA used extensively

Jnc: An OK start

2019-08-13T13:14:15Z

An OK start

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The '''PDP-11 Absolute Loader''' is a short [[program]] designed to be loaded into the [[main memory]] of a [[PDP-11]] by the [[PDP-11 Bootstrap Loader]] (most [[object code|binary]] copies of the absolute loader are in the format expected by the bootstrap loader); it in turn is then run to load some other program into the machine.

The bootstrap loader is designed to be absolutely as short as possible (since it is manually toggled in via the [[switch register]]), and therefore has limitations, including how long a program can be loaded using it, and where in memory that program can be placed; the absolute loader relaxes both of those limitations.

The initial version of the absolute loader had three different operating modes, controller by the contents of the switch register. If '0', it operates 'normally'. If '1', each block is loaded immediately following the previous block (i.e. the [[address]] in the block header is ignored). If 'xxxxx1' (in binary), the block is loaded at the address given in the high bits. For later PDP-11's, which did not have a switch register, a modified version is reequired.

==Absolute Loader format==

The format of absolute loader input is that it consists of an arbitrary number of blocks, each of which has the following format:

* Leader (all [[byte]]s 0)
* Start marker (1 byte, value '1')
* Pad (1 byte, value '0')
* Low byte count (1 byte)
* High byte count (1 byte)
* Low load address (1 byte)
* High load address (1 byte)
* Program to be loaded
* [[Checksum]] (1 byte)

The optional leader is because the absolute loader was originally for loading programs held on [[paper tape]]. The byte count includes the header starting with the 'start marker' (i.e. 6 bytes), but does ''not'' include the checksum byte.

The checksum is the low-order byte of the negation of the sum of all the bytes in a block; when all the bytes of a block, including the checksum, are added together, the low-order byte of the result should be zero (if the block has been read correctly).

A block of length 6 (i.e. only the header) marks the end of the input. If the address is odd, the absolute loader [[halt]]s; if even, it [[jump]]s to the address given in the last block.

==Further reading==

The Absolute Loader is documented in some detail in the "paper tape software programming handbook" (DEC-11-GGPB-D), pp. 6-8 to 6-12.

{{PDP-11}}

[[Category: PDP-11s]]

@@ Line 29: / Line 29: @@
 ==Further reading==
-The Absolute Loader is documented in some detail in the "paper tape software programming handbook" (DEC-11-GGPB-D), pp. 6-8 to 6-12.
+Usage of the Absolute Loader is documented in some detail in the "PDP-11 Paper Tape Software Programming Handbook" (DEC-11-GGPB-D), pp. 6-8 to 6-12.
 [[Category: PDP-11 Software]]
 [[Category: PDP-11 File Formats]]

@@ Line 31: / Line 31: @@
 The Absolute Loader is documented in some detail in the "paper tape software programming handbook" (DEC-11-GGPB-D), pp. 6-8 to 6-12.
-[[Category: PDP-11s]]
+[[Category: PDP-11 Software]]
 [[Category: PDP-11 File Formats]]

@@ Line 1: / Line 1: @@
 The '''PDP-11 Absolute Loader''' is a short [[program]] designed to be loaded into the [[main memory]] of a [[PDP-11]] by the [[PDP-11 Bootstrap Loader]] (most [[object code|binary]] copies of the absolute loader are in the format expected by the bootstrap loader); it in turn is then run to load some other program into the machine.
-The bootstrap loader is designed to be absolutely as short as possible (since it is manually toggled in via the [[front panel]], and therefore has limitations, including how long a program can be loaded using it, and where in memory that program can be placed; the absolute loader relaxes both of those limitations.
+The bootstrap loader is designed to be absolutely as short as possible (since it is manually toggled in via the [[front panel]]), and therefore has limitations, including how long a program can be loaded using it, and where in memory that program can be placed; the absolute loader relaxes both of those limitations.
 The initial version of the absolute loader had three different operating modes, controlled by the contents of the [[switch register]]. If '0', it operated 'normally'. If '1', each block is loaded immediately following the previous block (i.e. the [[address]] in the block header is ignored). If 'xxxxx1' (in binary), the block is loaded at the address given in the high bits of the switch register. For later PDP-11's, which did not have a switch register, a modified version is required.

@@ Line 3: / Line 3: @@
 The bootstrap loader is designed to be absolutely as short as possible (since it is manually toggled in via the [[switch register]]), and therefore has limitations, including how long a program can be loaded using it, and where in memory that program can be placed; the absolute loader relaxes both of those limitations.
-The initial version of the absolute loader had three different operating modes, controller by the contents of the switch register. If '0', it operates 'normally'. If '1', each block is loaded immediately following the previous block (i.e. the [[address]] in the block header is ignored). If 'xxxxx1' (in binary), the block is loaded at the address given in the high bits. For later PDP-11's, which did not have a switch register, a modified version is reequired.
+The initial version of the absolute loader had three different operating modes, controlled by the contents of the switch register. If '0', it operated 'normally'. If '1', each block is loaded immediately following the previous block (i.e. the [[address]] in the block header is ignored). If 'xxxxx1' (in binary), the block is loaded at the address given in the high bits of the switch register. For later PDP-11's, which did not have a switch register, a modified version is required.
 ==Absolute Loader format==
@@ Line 24: / Line 24: @@
 A block of length 6 (i.e. only the header) marks the end of the input. If the address is odd, the absolute loader [[halt]]s; if even, it [[jump]]s to the address given in the last block.
 ==Further reading==