ITS SBLK format
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.
Contents
CREATING AN SBLK FILE
The HACTRN $Y and MACDMP/DSKDMP D$ command writes a core image file in SBLK format.
OVERVIEW
An SBLK file consists of five sections:
- RIM10 loader.
- Simple blocks.
- Start instruction.
- Symbol table.
- 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.
