Method and apparatus for packing digital data

Abstract

A system device of a PC, XT or AT type computer having an ISA bus is provided with a dynamic 32-bit bus by packing circuits or PACs (142, 152) located on user add-on cards. Each PAC includes a state machine (200) which controls four tag registers (210, 211, 212, 213), four input data registers (220, 221, 222, 223), four output data registers (240, 241, 242, 243), and an output multiplexer (250). The four tag registers are for storing a byte-high enable signal BHEN and system address bits SA[1:0] associated with bytes, words, and doublewords presented to the PAC during bus write cycles. The four input data registers are for storing the bytes, words, and doublewords presented to the PAC during bus write cycles. These bytes, words, and doublewords are steered to appropriate bit positions in the input data registers by four steering circuits (214, 215, 216, 217), which are controlled by the platform type signal CR2B2.sub.-- 1 and by the output of a decoder decoding the outputs of the tag registers. The four output data registers are for storing outputs of the four input registers received through an encoder (234). The output multiplexer is for selecting an output of one of the output registers. For AT-type platforms, up to two adjacent words are packed into one 32-bit doubleword for output. For PC and XT type platforms, up to four adjacent bytes are packed into one 32-bit doubleword for output. The PACs are also compatible with computers having EISA or PCI buses. During 32-bit EISA or PCI bus cycles, the input doubleword is furnished as the output doubleword.