Friday, 16 September 2016

Y NO Z80+editable chars?


Mode Z80 Non-Z80 (6502, 6800)
Memory Mapped
Character Display
Enterprise, Sharp MZ series, Aquarius, Laser 200 VIC-20, C64, Panasonic JR200, Atari 8-bit, Commodore Plus/4, Oric Atmos
Memory Mapped
Character Set
Enterprise, Sharp MZ-800(?) VIC-20, C64, Panasonic JR200, Atari 8-bit, Commodore Plus/4, Oric Atmos
Memory Mapped
Bitmap Mode
Enterprise, ZX Spectrum, Amstrad, ZX81, SAM Coupé, Sharp MZ VIC-20, C64, Panasonic JR200, Atari 8-bit,, Commodore Plus/4, Oric Atmos
VDP MSX 1/2, Sega SC-3000, Coleco Adam, Memotech MTX, Tatung Einstein


Character displays proved to be very effective for 8-bit computers. Intuitive to program, less wasteful of memory and with high potential for optimizing graphics for your chosen approach.
loop:INC $0400,X;INC $0500,X;INC $0600,X;INC $0700,X;INX;JMP loop
What the above table highlights is that there seem to be very few computers combining the Z80 processor with a memory mapped character display with editable character set. I've only heard that Enterprise, the swansong of the 8-bit era, might have such modes. Sharp MZ-800 has something called the Programmable Character Generator, which I think does the trick too. I'm pretty sure that another latecomer, SAM CoupĂ©, does not have a true text mode. The point is that none of the popular Z80 computers had the feature.

Some websites list Sinclair-style pseudo-text bitmap modes as "text modes" too, so it can be confusing. The accurate distinction between pseudo-text, fixed character set modes and fully editable character set modes is not made often enough.

Why were there so few Z80-based memory mapped character graphics? Surely, if the 6502 can whizz user-defined characters around the screen, the Z80 could whizz considerably more?

I don't know. Maybe the engineers thought the Z80 was in any case powerful enough to handle bitmap graphics (a very marketable feature back then). Or, there may be some bus-architectural reason why combining Z80 with a sophisticated character display would be difficult or result in something that brings complications for the programmer or additional contentions with the other chips. I'm especially wondering this as there are Z80-computers with a char display (Aquarius, Sharp series, Laser) but without editable characters. A full 256-character set would have eaten 2K of RAM though.

In the table I've also included some VDP-based computers. It was close to being a standard solution and one can suppose at some point it was easier to design a Z80+VDP computer than start creating new video chips. There seem to be no 6502+VDP computers either, which might have been a poor combo.

1 comment:

  1. There were VDP boards for the Apple II that used the Ti VDP. I've often thought that it was synchronous nature of the 6502 and the fact that you could easily write too quickly to the VDP before it is ready for the next operation, but somehow these Apple Ti boards overcame that obstacle.

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