(from Your Spectrum 4, Jun.1984)





DUMPS OF DISTINCTION



Frustrated at producing tiny screen dumps on your

full-sized printer? So was Andrew Pennell - until

he developed this clever little machine code

utility, that is!





If you're fortunate (ie. rich) enough to have an

Epson RX8O or FX8O connected to your Spectrum,

then this is the screen dump program for you.



Most printer interfaces are supplied with a high

resolution dump routine, which produces a ZX

Printer-sized copy of the screen. However, the

copies lack the different colours of the original,

and the colour system on the Spectrum can produce

weird results when copied because only the screen

bitmap is examined, and the colour information is

ignored.



Nothing's ignored in this colour screen dump.

However, not having the wherewithal for a full

seven-colour printer, I decided to write a screen

dump routine for my Epson that printed a large

picture with different shades simulating different

pixel colours; the following program is the

result, producing an 11-by 6-inch copy.



The program works on either the 16K or 48K

Spectrum, as it moves RAMTOP down to address

32349; 48K owners cannot use their extra 32K of

memory. For those with the inclination, the

routine can be modified to run from address 65118,

by changing all '205,20,127' sequences to

'205,20,255', and by changing '33,244,126' to

'33,244,254'.



Listing 1 shows the Basic loader for those with

the Hilderbay interface; note that you must load

the 'mini- software' first. For those with a

Kempston interface, add the lines in Listing 2

and, for those with a Kempston E interface, add

the lines in Listing 3. If you've got none of

these interfaces, you'll just have to modify the

machine code yourself - details to follow. And

equally important, if your printer requires Line

Feeds after Carriage Returns, add the line:



230 POKE 32478, 205: POKE 32479,20: POKE 32480,127



Once you've entered and RUN the loader, save the

routine to tape with SAVE "col copy" CODE

32350,250 - and the Basic loader after it, in case

of a crash. Then load a SCREEN$, and type

RANDOMIZE USR 32350. If your printer starts

printing, you've probably got the code right. If

it doesn't, or if the system crashes, then there

must be a mistake and you should re-load the Basic

and check it against the listings.



I think the best screen dumps are those from the

Ultimate programs, and some of these adorn this

article. Unfortunately, they all contain a lot of

black, so you'll need a new(ish) ribbon for best

results. If you're using fan-fold paper 11-inches

in length, position the perforations just above

the print head before the dump - otherwise the

next perforation will appear in the middle of the

dump. if you break in the middle of it, you may

find that your printer acts strangely, because

it's in bit mode; the best way to clear it is by

switching the printer off, then on again.





MODUS OPERANDI



The screen is copied sideways, and consists of a

176 x 256 grid of pixels. If each screen pixel is

represented by a three- by- three grid on the

printer, then this points to a printer resolution

of 528 x 768. The former figure rules out the old

Epsons (MX-type and assorted look-alikes) with

their resolution of only 480 dots per line. The

new Epsons have a resolution of 640 dots per line

in bit image mode 4, and that's why we're using

them here. As well as this, the vertical Line Feed

distance has to be changed to three dots, which is

3/72 of an inch, and is changed with the ESC "A"

command.





CRACKING THE CODE



The full assembler listing is shown in Listing 4,

printed from the Hisoft GENS assembler, which uses

'#' to denote hex numbers, and '%' for binary

numbers. It also supports conditional assembly,

which I have used to determine the setup and

output routines for different interfaces. If your

assembler does not support conditional

assembly,just leave out all mnemonics that refer

to interfaces other than your own. If you have a

different interface, substitute your own set-up

routine at START, and your own output routine at

OUTCH. Note that OUTCH must preserve the values of

HL and BC.



The copy routine proper starts at BEGIN, which

firstly resets the Line Feed distance to 3/72 of

an inch and zeros C - which is used as a counter

for the X co-ordinate. Next the printer is put

into graphics mode 4, and the B reg (which is the

Y counter) is set to zero. The ROM routine at 22AA

is called, which calculates screen location HL and

bit position B from the screen co-ordinates in B

and C, and the pixel colour calculated from the

attribute file. The relevant data is read from

TABLE and printed out, and this is repeated for

all 176 Y positions. After this, a Newline is sent

(and optionally a Line Feed), and all 256 X

positions are done. Finally, the Line Feed

distance is reset to 12/72 of an inch. The data at

TABLE contains the bit patterns for each colour.

They were originally chosen quite arbitrarily, but

I left them as they seem to produce good results.

If you don't like the shades produced, feel free

to change the data, though note that the fourth

byte for each colour should be zero. Also, beware,

Listing 4 was made with the Hilderbay interface

selected, so no object code was generated for

either of the Kempston interfaces.





The Basic loader for the Hilderbay interface.



Add these lines to the loader to use with a

Kempston interface.



Add these lines to the loader to use with a

Kempston E interface.



This listing is taken from the Hisoft GENS

assembler and uses # to denote hex numbers and %

for binary numbers. It also supports conditional

assembly.