IN that sense yes. Is it requried of the OS no. Does the OS need it, no. CP/M was minimal as it's day ram was expensive interms of cost, power and space. Was it short sighted, yes. Would it have been implmented right in 1975? No as terminals were just starting to have basic intelligence and maybe 2-3 years later what would the requirements be like? Many others too like Vedit. It was a one time task, not nice but compact. Whats more interesting is there was nothing to prevent a termcap file and later improved CP/M work alikes did exactly that and many more things. The bottom line is sans BIOS where do you put termcap in a OS thats only 5.5KB? As to unix, it was the big machine OS and was not going to run on a 256kb floppy. It was only popular in academic and research and far from mainstream till the mid 80s. Or the +80H (end of line on high bit set). Paper tape and Punch were rare in most CP/M systems and often unimplmented. I tend to implment the console and list fields (upper and lower two bits) completes to use the printer ports and second (and third serial if available). Andy Johnson-Laird The Programmers CP/M Handbook went far to extend and clarify both what the BIOS can do and more. Actually I'm running system with CP/M-V2.2 that page the BIOS and have multiple hard disks and a tpa in the 62-63k range. CP/M+ is not required to achive that. But both approaches need some kind of memory paging and the ram/rom and a BIOS to support it. The otehr issue is most developers didn't have a useful system interrupts or were time pressed enough to feel that it was worth it. By useful interrupts I mean.. most early S100 8080 machine if you pulled the interupt line the default was vector to 38H as that was RST7 (11111111b) which happend as a result of pull up resistors. that location is ued by DDT for trap. Early Z80s also did that. later 8080/8085 and Z80 systems implmented basic vectored interrupts so how you could use RST 2 through 6 and that meant resonable interrupt drivers wer possible. The Z80 SBCs like Ampro and others that used the Z80 peripherals all had the Z80 vectored system which was powerful and a bit intimidating to those that hand not used them before. As you can se the whole interrupt thing was not CP/M as a limiting factor but hardware or implementor understanding. For those that never used a really nice bios try a VT180, it didn't do two sided but those disks where just emerging at the time. It did implement interrupts with ring buffers for IO. The other thing was DMA. On S100 is was a timing and bus nightmare and took years to almost get right. Many of the single baord systems omitted it as it took space (8257 or later 8237 40 pin chips and a latch). It works fine and made useful systems. However it means the CPU is locked up for the duration of the transfer and cannot respond to interupts making for poor latency as floppies are slow. Again CP/M doesn't care how the transfer happens only that it does happen. So the fist system built with DMA was a real eye opener. First it allowed background activities to run faster and smoother like a line printer spooler. also interrupts could be used byy the disk system to say "ready" or "ready with error". Thats a lot of available CPU cycles. the biggest areas of change is that modem programs werent pausing for disk IO, they could fill a big (say 16k) circular buffer and the cpu can be processing interrupts for IO and disk to manage transfers rather than doing a lot of waiting in loops. It doesn't take a lot more code but the complexity and debugging is greater due to the near concurrent activities. Thats what I believe in too for dos. Yes thats true after NT flavored took over (NT, Win2k XP and likely vista). for win 3.1x it was dos with gui and for Win9x dos was very much there. I ahve Dos7 and dos8 which is extractable from win9x. Nothing worth reporting there. :) Allison