ITS SBLK format

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The ITS SBLK, or simple block format, was introduced with the Project MAC PDP-6 MIDAS assembler and adopted in the transition to time-sharing.


The HACTRN $Y and MACDMP/DSKDMP D$ command writes a core image file in SBLK format.


An SBLK file consists of five sections:

  1. RIM10 loader.
  2. Simple blocks.
  3. Start instruction.
  4. Symbol table.
  5. Start instruction.

RIM10 loader

An SBLK file traditionally starts with a paper tape RIM10 bootstrap loader ending with a 'JRST 1 instruction. The loader is now optional except for the JRST.

Simple blocks

The core image is stored in a series of zero-compressed blocks. A block starts with an AOBJN header:

0-17 18-35
-count address

After the header follows count word to be stored starting at location address'.

A simple block is ended by a checksum. The checksum includes the header word. A new checksum value is computed from the old checksum rotated left one bit, added to the word.

Start instruction

After the simple blocks comes a start instruction which must have bit 0 cleared. Conventionally this is a JRST, but sometimes JUMPA is used.

Symbol table

The symbol table is optional and may be empty.

The table is stored as simple blocks with the address as zero. There are alternating words of SQUOZE symbol names and 36-bit symbol values.

Repeated start instruction

After the symbol table there is a repeated start instruction.