P21Forth 1.02 includes two words to change the speed of I/O. If you have installed the ACT type parts on your kit for performing high speed I/O you can take advantage of this by changing to high speed I/O with the word FAST-I/O. The word SLOW-I/O will change the I/O words to use the slow speed I/O address space and the word FAST-I/O will change these words to the fast I/O address space.
The file SLOW.ROM contains a copy of the PCMCIA card image that will boot up and use slow speed I/O by default. If the P21Forth 1.02 SLOW.ROM file can also be burned into a UVPROM and installed on kits with either the slow or fast I/O chips. If the fast I/O chips are installed you can take advantage of this with the command FAST-I/O. The ROM containing the slow I/O operations can be booted and run on a system with either the slow or the fast I/O chips installed.
The file FAST.ROM constains a copy of P21Forth that will boot up in the default fast I/O speed mode. You can also switch between slow and fast I/O in this version. You cannot use the fast I/O speeds with the 74LS parts in the kit, they are not fast enough.
The use of fast I/O will make the video signal much less noisy at lower operating voltages. Above 5.8 volts there will be little video noise, with either fast or slow I/O, but below this voltage there will be video jitter with slow I/O. This is because the slow I/O operation, or access to the UVPROM or PCMCIA card via BLOCK words is a very slow operation. The access to these spaces requires 250ns. If this is combined with an off page instruction fetch in DRAM and some instruction execution the video processor may not get access to memory again before it has missed a pixel. If the video processor misses a pixel in real time the result is video jitter. Fast I/O will only take 40ns, so there is more memory bandwidth available to the video coprocessor on the MuP21.
P21Forth 1.00 did not have the correction to place a NOP before A! instructions that did not appear in the first slot. This caused it to not run on the plastic MuP21 chips at lower voltages. On plastic MuP21 chips P21Forth 1.00 would not run below about 5.4V.
P21Forth 1.01 did have the NOP before most A! instructions not appearing in the first instruction slot, and would run down to about 4.9V. P21Forth 1.02 uses an improved EXIT routine supplied by Raul Miller, and in doing so also replace the old EXIT routine which did not have a NOP before an A! . This has improved the speed of P21Forth 1.02 over 1.01 and also reduced the lowest voltage at which P21Forth 1.02 will run to about 4.8V.