modern serial terminal

On 5 Nov 2007 at 6:08, Steve Stutman wrote: Please, you miss my point, which is that it's not the comm interface on a PC that's the problem; it's the display--and that there are ways to handle it so that it's not a problem. Many PC display controllers have a 6845 sort of video controller with shared access to local memory--and a register in the controller that indicates where within video memory that display is to begin. When the controller reaches the end of video RAM, it simply wraps around to the beginning. Most PC display software doesn't fool with the start address, preferring to scroll by shifting all but one line's worth of bytes in the display by one line. This is a very time-consuming job and usually results in data loss if a steady stream of data with plenty of linefeeds is encountered. Instead of shifting the data, simply change the starting address in the controller register. The only downside is that you must be aware of the starting address--a location on the screen does not have a static mapping to memory. But in the case of terminal software, this is a small price to pay. If a Z80-based terminal has to scroll by using LDIR's to shift lines of characters and screen attributes, it will fall behind too-- regardless of the quality of its silicon implementation. It's more of a problem on chipsets such as the Intel 8275 CRTC, where refresh is accomplished by DMA requests through something like an 8257 DMA controller with common RAM shared by the processor. Unless the designer has included "wraparound" addressing, scrolling becomes a problem. It can be ameliorated somewhat by allocating a large display buffer (2x-3x screen size) and moving the starting DMA address to scroll. When the end of the buffer is about to be reached, one can copy a screen's worth back to the start of the display buffer. This can be spread over the time it takes to fill the final end-of-buffer screen, but, but it's a lot more difficult to manage well. But then, I'd rather perform do-it-yourself root canals than use an 8275 CRC. What a piece of work. Cheers, Chuck