318 Part 2 ½ Regions of Computer Space Section 3½ Concurrency: Single-Processor System

ing and word-slice accesses for STARAN input/output (I/O), the data are stored in a multi-dimensional access (MDA) memory (Fig. 2).¹ It has wide read and write busses for parallel access to a large number (256) of memory bits. The write mask bus allows selective writing of memory bits. Memory accesses (both read and write accesses) are controlled by the address and access mode control inputs; the access mode selects a stencil pattern of 256 bits, while the address positions the stencil in memory.

For many applications, the MDA memory is treated as a square array of bits, 256 words with 256 bits in each word. The bit-slice access mode (Fig. 2a) is used in the associative operations to access one bit of all words in parallel, while the word access mode (Fig. 2b) is used in the I/O operations to access several or all bits of one word in parallel.

The MDA memory structure is not limited to a square array of 256 by 256. For example, the data may be formatted as records with 256 8-bit bytes in each record. Thirty-two such records can be stored in an MDA memory and accessed several ways. To input and output records, one can access 32 consecutive bytes of a record in parallel (Fig. 3a). To search key fields of the data, one can access the corresponding bytes of all records in parallel (Fig. 3b). To search a whole record for the presence of a particular byte, one can access a bit from each byte in parallel (Fig. 3c).

The MDA memories in the STARAN array modules are bipolar. They exhibit read cycle times of less than 150 nsec and write cycle times of less than 250 nsec.

¹The passage beginning with this paragraph is from Batcher [1974].