Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!ncar!midway!ellis.uchicago.edu!jcav From: jcav@ellis.uchicago.edu (john cavallino) Newsgroups: comp.sys.mac.hardware Subject: Re: LC video vs. SI video... (really built-in video in general) Summary: how it works Keywords: video built-in Message-ID: <1991Mar6.011546.19535@midway.uchicago.edu> Date: 6 Mar 91 01:15:46 GMT References: <1991Feb26.134342.25399@magnus.ircc.ohio-state.edu> <1991Mar6.094536.125@otago.ac.nz> Sender: news@midway.uchicago.edu (News Administrator) Organization: University of Chicago Lines: 29 There are three (3) built-in video methods employed by Apple in Mac models. 1) The Mac IIci and IIsi: The frame buffer ALWAYS starts at absolute address 0, which is the first address in SIMM bank A. This is where the video contoller always looks. Depending on the screen depth selected in the "Monitors" control panel, there is a variable amount of RAM left unused in bank A. The MMU maps this RAM, along with any RAM in bank B, so that it appears contiguous in virtual space, starting at virtual address 0. It also remaps the frame buffer to appear in the address space corresponding to a NuBus card. The reason some memory is slow and some not slow is that sometimes the absolute address falls in bank A, which suffers contention delays with the video controller. If the absolute address falls in bank B, there is no contention and no delay. This is also why it is best to put expansion RAM in bank B first. 2) The Mac LC and the Mac SE/30: The video in these systems works differently, in that the frame buffer is stored in dedicated video RAM chips, not in the main SIMM memory. 3) The "classic" machines (128,512,Plus,SE,Classic): The video in these systems is similar to method 1, in that the frame buffer is stored in regular memory, but in these machines ALL memory suffers from contention delays (less so in the SE and Classic). -- John Cavallino | EMail: jcav@midway.uchicago.edu University of Chicago Hospitals | USMail: 5841 S. Maryland Ave, Box 145 Office of Facilities Management | Chicago, IL 60637 "Opinions, my boy. Just opinions" | Telephone: 312-702-6900