per
Veteran Member
Yes, you read correctly; I wish to underclock an AT-class system.
The reason for this is that some games were especially timed for the 8MHz Turbo XTs (with an average performance of about 0.55 MIPS), and will either run to fast on more recent AT machines (+0.7 MIPS), or too slow in an original 4.77MHz XT (~0.33 MIPS). It is to note that in some cases, the game itself is not running off speed, but all the animations are off. Of course I can allways try to obtain an XT-clone or an early AT, but the problem I have is that I am short of space, and time to get it working if it turns up in lousy condition.
The AT machine I got is an IBM XT/286, running at 6MHz and without any waitstates for the RAM. This makes it preform at 0.9 MIPS, which is clearly noticable in some of the titles I have been trying out with. I have calculated that if I want to get it down to the appropriate MIPS value, I need to install a 7.36MHz crystal (for a clock rate of 3.68MHz for the CPU, and either 2.76MHz or 4.77MHz for any installed FPU) in place of the 12MHz crystall already in there.
My question is, as listed in the title; How far can you underclock a 286? The registers of the regular 8088 becomes unrelaiable below 2MHz (due to dynamic memory used for the registers), so I expect the AT to have a minimum clock too. I also fear it may cause incompabilities with timing-critical hardware and/or software, and that the speed check within the BIOS may fail. I know there has been instructions somewhere about how to patch the BIOS, but I have not been able to find instructions about this online.
The reason for this is that some games were especially timed for the 8MHz Turbo XTs (with an average performance of about 0.55 MIPS), and will either run to fast on more recent AT machines (+0.7 MIPS), or too slow in an original 4.77MHz XT (~0.33 MIPS). It is to note that in some cases, the game itself is not running off speed, but all the animations are off. Of course I can allways try to obtain an XT-clone or an early AT, but the problem I have is that I am short of space, and time to get it working if it turns up in lousy condition.
The AT machine I got is an IBM XT/286, running at 6MHz and without any waitstates for the RAM. This makes it preform at 0.9 MIPS, which is clearly noticable in some of the titles I have been trying out with. I have calculated that if I want to get it down to the appropriate MIPS value, I need to install a 7.36MHz crystal (for a clock rate of 3.68MHz for the CPU, and either 2.76MHz or 4.77MHz for any installed FPU) in place of the 12MHz crystall already in there.
My question is, as listed in the title; How far can you underclock a 286? The registers of the regular 8088 becomes unrelaiable below 2MHz (due to dynamic memory used for the registers), so I expect the AT to have a minimum clock too. I also fear it may cause incompabilities with timing-critical hardware and/or software, and that the speed check within the BIOS may fail. I know there has been instructions somewhere about how to patch the BIOS, but I have not been able to find instructions about this online.
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