Common Programming Models

Murrough Landon - 6 July 2000

Aims

• Reasonably coherent programming models for all of our modules
• Similar notation used in documentation of programming models
• Reduce complexity and variation
• Easier life for small group of software writers!

Guidelines

• All registers are readable (no write only registers). The register bits should have the same meaning on read and write accesses.
  • All status registers shall be read only
  • All control registers shall be read/write
  • Reading back a register will generally return the last value written (there may be some write only reset bits which read back zero)
• Attempts to write to read only registers or to undefined portions of registers will result in the unmodifiable fields being left unchanged.
• It is illegal to write a value to a register which the module itself is able to modify at the same time. [This may be a bit strong!].
• When the address space occupied by the module is accessed, it will always respond with a handshake to avoid a bus error.
• The power-up condition of all registers will be zero unless otherwise stated in the documentation.

Notation

• A byte is always an 8 bit field; a word is always a 16 bit field; a longword is always a 32 bit field.
• All offsets specified as byte offsets.
• Setting a bit-field means writing a 1 to it, clearing it means writing a 0.
• RO = read only; RW = read/write.

Module ID

• Common module ID format for all modules. The DSS defined a 32 bit format, ie two 16 bit registers:
  • High word: module type
  • Low word bits 0-7: serial number (1-255)
  • Low word bits 8-11: revision (1-15)
  • Low word bits 12-15: unused, read as zero
• Traditionally located at the module base address (convenient for scanning VME bus).