maxtherabbit
Veteran Member
I've got a pretty neat 486 board here. Built in 1991, it uses a Zymos POACH chipset (designed for 286 systems) grafted to a 486 processor and cache with the help of a bunch of PAL/GALs and discreet logic.
My issue with it is that despite using an ET4000AX card which will manage >4MB/sec on a 286 system, I'm only able to get about ~2MB/sec throughput to video memory. This cripples an otherwise awesome board for gaming performance. So I am on a quest to improve the situation.
The POACH, being a 286 chipset, is designed to run at wire speed and does not offer any type of bus wait states or recovery cycles. Therefore, I can assume that the bus slowness is a factor of something in the glue logic. So far after conducting a brief survey of the components, the only thing that stands out at me is this: a Dallas DS1000-50 silicon delay line. I can't really think of another good reason for this to be present, so I'm considering removing it and shorting the input to the tap. Thoughts?
I've already started a identical thread on vogons, so I'll go ahead and address the first few points that came up over there.
1) Does the POACH have any hidden configuration options? No. As far as I can tell from the datasheet, it has no configuration registers whatsoever.
2) Does the BIOS have any hidden configuration options? Doubtful. I'll attach a dump for the curious, but it's a bog standard pre-color AMIBIOS with extra options only for shadow on/off and cache on/off. These options must be physically implemented in one of the many PAL/GALs, so there is indeed the possibility that more configuration bits exist in register(s) exposed to an IO port.
3) What speed is the ISA bus running at? It has its own oscillator of 16MHz, which is divided by 2 in the POACH. I've verified the ISA slots get a steady 8MHz on my scope.
My issue with it is that despite using an ET4000AX card which will manage >4MB/sec on a 286 system, I'm only able to get about ~2MB/sec throughput to video memory. This cripples an otherwise awesome board for gaming performance. So I am on a quest to improve the situation.
The POACH, being a 286 chipset, is designed to run at wire speed and does not offer any type of bus wait states or recovery cycles. Therefore, I can assume that the bus slowness is a factor of something in the glue logic. So far after conducting a brief survey of the components, the only thing that stands out at me is this: a Dallas DS1000-50 silicon delay line. I can't really think of another good reason for this to be present, so I'm considering removing it and shorting the input to the tap. Thoughts?
I've already started a identical thread on vogons, so I'll go ahead and address the first few points that came up over there.
1) Does the POACH have any hidden configuration options? No. As far as I can tell from the datasheet, it has no configuration registers whatsoever.
2) Does the BIOS have any hidden configuration options? Doubtful. I'll attach a dump for the curious, but it's a bog standard pre-color AMIBIOS with extra options only for shadow on/off and cache on/off. These options must be physically implemented in one of the many PAL/GALs, so there is indeed the possibility that more configuration bits exist in register(s) exposed to an IO port.
3) What speed is the ISA bus running at? It has its own oscillator of 16MHz, which is divided by 2 in the POACH. I've verified the ISA slots get a steady 8MHz on my scope.