tdl (03/16/83)
With due respect to Henry Spencer I feel that I must correct some of his statements regarding the 68000. He is correct in saying that the 68000 is basically 16 bits wide; however, his explanation of the segmented bus is incorrect. The datapath of the 68000 is divided into three pieces, each of which has two busses, address and data, running through it. Six busses total. There are muxes which can be switched so that all address busses are connected and all data busses are connected. The three sections of the datapath are the data section (includes low 16 bits of all data registers and ALU), the "low" section (contains the low 16 bits of address registers and the low half of the Address Adder(AAU)), and the "high" section (contains high 16 bits of all address and data registers and the upper half of the AAU). Theoretically they could do 6 16 bit transfers simultaneously, but in looking through the microcode I don't remember seeing more than three transfers at a time. The "low" and "high" sections can be cascaded to provide a 32 bit arithmetic unit for address calculations. 32 bit data calculations must be done in two passes through the ALU. For the masochists out there, you can learn more than you ever wanted to know about the 68000 by reading Motorola's patents on it. They are available for some nominal fee (~ one dollar) from the Office of Patents and Trademarks in Arlington. The relevant patents are: 1 - #4,307,445 "Microprogrammed Control Apparatus Having a Two Level Control Store for Data Processor", Tredennick, et al. First design of 68000 which was scrapped? 2 - #4,296,469 "Execution Unit for Data Processor using Segmented Bus structure", Gunter, et al. All about the 16 bit data path 3 - #4,312,034 "ALU and Condition Code Control Unit for Data Processor", Gunter, et al. Boring. 4 - #4,325,121 "Two-Level Control Store for Microprogrammed Data Processor", Gunter et al. Bonanza! Full of block diagrams and everything you ever wanted to know. Includes complete listing of microcode with Tredennick's "hardware flowcharts". Hope this clears things up. Tom Lovett BTL Holmdel harpo!houxk!tdl 201-949-0056