INMC 80 News

  

September 1980 – January 1981, Issue 2











Page 15 of 59











Not a bad little trick that, note the use of the warm boot jump to tell us what size the CP/M system is. This little trick has other uses. For instance how about a tape read routine in generalized Nascom format. The problems come thick and fast with that one. The main problem is to duplicate TXl out of NAS-SYS. This is the one that goes away, displays the header and checks the checksum. Because of the length of the CRT routine it barely makes it at 2400 BAUD at 2MHz on a Nascom as it is. Now to do the same thing under CP/M takes even longer, not only does the poor little byte to be displayed have to fight its way through the CRT routine in CBIOS (thats already as tortuous as the one in NAS-SYS), but it has to go through all the CP/M FDOS checks first. Believe me, it only just makes it at 1200 BAUD at 4MHz with no wait states. So how to bypass the FDOS (‘cos we already know the data is correct)? Well it would be a simple matter to call CONOUT in the CP/M, the only trouble is, it moves about with different size systems, so we have to find out where it is first. We already know where the system is located because the warm boot jump at 0000H tells us, we also know that CONOUT is offset from WBOOT by a fixed amount, so we can calculate its address. Now to call it. We use what is known as a ‘fake call’. Find out where the routine is to return to, push that location onto the stack and then jump to the routine, the address of which we have just calculated. Having completed the routine, the RET at the end pops the last address off the stack (as normal), and returns. Provided nothing happened to upset the stack, this will be the return address. The CRT routine looks like this:


This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 15 of 59