Fall 2008, Oct. 31 ELEC / The IBM360 and Tomasulo's Algorithm Joel D. Hewlett

The IBM360 The IBM 360 The IBM 360 is a family of mainframe computers introduced by IBM in Fall 2008, Oct. 31 ELEC /

Key Features Affordable –Upgradeable Common Instruction Set Microcoded CPU Unlimited Storage Instruction Level Parallelism –Tomasulo's Algorithm Fall 2008, Oct. 31 ELEC /

Common Instruction Set The IBM 360 used a common expandable instruction set, which meant: –Reduced financial risk –Affordable upgrades –Forward compatibility –Simple transition from one system to the next Fall 2008, Oct. 31 ELEC /

CPU Microcoding Facilitated the use of a common instruction set for the entire IBM 360 line. Also allowed emulation of instruction sets from some non-IBM 360 machines. Fall 2008, Oct. 31 ELEC / Fall 2008, Oct. 31 ELEC /

Unlimited Storage Expandable Memory –Disk –Drum –Tape (low cost) No Reprogramming Fall 2008, Oct. 31 ELEC / Fall 2008, Oct. 31 ELEC /

A short aside... “Big, fast disk drives were one of the strengths of IBM. In 1973, the big mainframe disk drive was model : 400 MB for $111,600 or $279/MB. By 1980, you could get the 3380: 2.5GB for $87,500 or $35/MB. DRAM prices were dropping, too: In 1979 the price was cut from $75,000/MB to $50,000/MB.”

Tomasulo's Algorithm In 1967 IBM set out to design a model of the IBM 360 capable of non-sequential execution of instructions. Problem: in order to maintain the compatibility with the rest of the IBM 360 line, the IBM 360/91 needed to handle non- sequential execution without requiring changes to existing code. Solution: Tomasulo's algorithm.

Tomasulo's Algorithm Tomasulo's algorithm handles dependencies on a hardware level. No changes to software are required. Key Features: –Reservation Stations (RS) –Common Data Bus (CDB) Fall 2008, Oct. 31 ELEC / Fall 2008, Oct. 31 ELEC /

Reservation Stations Set of registers (control,sink,source) associated with each ALU unit. –Control register: identifies the operation to be performed (multiply/divide add/subtract). –Sink register: register where result is to be stored. –Source register: holds operands used in computation.

Common Data Bus The CDB is connected to all execution units. When the result from an operation is available, it is placed on the CDB. Any dependent operations may read the result from the CDB into their source registers.

3-Stage Process Stage 1: Issue –Load instruction unto a free RS. Stage 2: Execute –Once all source dependencies are resolved the operation in the corresponding RS is executed. Stage 3: Write –After execution, the result is written to the CDB. –In the case of a store, result is also written to memory.

Data Paths

Tomasulo's Algorithm RSs inherently resolve structural hazards. The CDB solves dependencies –By using CDB, the RSs automatically perform register renaming!

Impact Tomasulo's algorithm was the first commercial use of instruction level parallelism. The algorithm is still used today. All superscalar processors are based on Tomasulo's hardware algorithm. –Power PC, Intel, AMD

References