How was text handled on the Amstrad CPC 464?

Question

The three official video modes described on the CPC wiki are all bitmap modes, and this page mentions that the "screen is bitmapped. You must draw/erase your own sprites and text."

So what was the standard way of generating text on the screen, scrolling it, and so on? Were there ROM routines that most programs used, or was it just left up to the individual programs to do it with their own code? Was there any special hardware support to help with text and, particularly, scrolling? Were there standard text "modes," used by most programs and, if so, what were they?

Also, since the CPC was normally sold with a monitor, were text modes with more than 40 columns per line frequently used? I am guessing that with a monitor (particularly the monochrome one) the usual color vs. resolution issues on TVs could be avoided and so 64, 71 or even 80 column modes might be common, especially for business software.

Answer 1

All text is painted as bitmaps.

The highest-resolution built-in mode is 640x200 pixels, so that provides an 80x25 text mode. 80-column modes were used in business software (e.g. DBase II) and even in the CP/M 2.2 and 3.0 OS.

The CPC uses a CRTC6845, which is wired up atypically to create linear scan lines but nevertheless can still hardware scroll in multiples of 8 lines. So a hardware text scroll is available.

Firmware routines are provided to establish text windows, move the cursor, set colours, optionally set a custom font, and to paint text. So they're there if you want to use them.

The disk-based CPCs came with an implementation of CP/M that emulates the control codes of a Zenith Z19.

On clarity, I grabbed the following from a Youtube video demonstrating the 6128 hardware showing 80-column text on a real CPC:

So you can see that the screen is very sharp at that resolution; the colour screen received just as sharp a signal as the green screen shown, being a full RGB connection. But see @rwallace's comment re: dot pitch; the best video evidence I could find is unfortunately a glancing shot:

Though I think that at least establishes the absence of most of the standard computer-with-a-TV artefacts, at least for the era when only France could boast wide adoption of Peritel/SCART.

Answer 2

To elaborate on drawing text on Amstrad CPC and its screen addressing:

1. The first row of every character position on screen is at the memory address range from &c000 to &c7ff, and thus can be calculated as &c000 + y * 80 + x.

FOR I=&C000 to &C7FF: POKE I, &FF: NEXT

2. The subsequent lines of every character are &800 bytes apart from each other. For instance, each address line for the character at &c000 is calculated with &c000 + y * &800.

With this information, you can render text using a routine like this:

10 MODE 2
20 DATA &18,&3c,&7e,&ff,&ff,&7e,&3c,&18  : ' Load some 8 by 8 bitmap data
30 INPUT "x pos";x :INPUT "y pos";y
34 '
35 ' Text render begin
36 '
40 addr = &C000 + y * 80 + x             : ' Calculate screen start address
60 FOR y0 = 0 TO 7                       : ' For 8 pixels high
70 READ p                                : ' Read pixel data
80 POKE addr, p                          : ' Poke it at screen address
90 addr = addr + &800                    : ' Calculate next line's screen address
100 NEXT
105 '
106 ' Text render end
107 '
110 END

For more information you can have a look at my personal notes about Amstrad CPC display generation (and some more) here:

https://neuro-sys.github.io/2019/10/01/amstrad-cpc-crtc.html