Scrolls

Scrolls





* T A Bratley * Grimsby, South Humberside





Here is a set of eight versatile, easily used and interes-


ting scroll routines which can bve added to your Basic


programs to clear all or part of a screen in one of 8


directions.


The routines scroll one character square at a time in the


following compass directions: N, S, E, W, NE, NW, SE, SW.


The old character square is blanked out and its attribute


is replaced with one of your choice.


The difference between these and previously published


routines is that any section of the screen can be scrolled


in any direction and leave any colour attribute behind.


This includes the bottom two lines.


There are only five parameters you have to supply in


order to define the section of the screen to be scrolled,


and below are the addresses you need to poke for both 16K


and 48K machines (16 in brackets).


POKE 64000 (31232), h (1 to 24)


POKE 64001 (31233), w (1 to 32)


POKE 64002 (31234), r (0 to 23)


POKE 64003 (31235), c (0 to 31)


POKE 64004 (31236), a (attribute)


This defines a box of height h, width w, with a top left-


hand corner at r,c (where r and c are the normal PRINT AT


coordinates), to be replaced with a new attribute a.


Height and width must always be at least one because a


box must have two dimensions, and care must be taken to see


that the box does not exceed the screen limits.


The attribute is calculated in the usual way by multi-


plying the paper colour by eight and then adding the ink


value. If bright is required, add a further 64 to the total


so far, and finally if flash is required add a further 128.


After poking the values for machine code routines to use


you will then have to select whichever scroll direction you


require.


The table below gives you the addresses for both


machines:


DIRECTION 16K 48K


N 31241 64009


S 31253 64021


E 31265 64033


W 31277 64045


NE 31289 64057


SE 31303 64071


SW 31317 64085


NW 31331 64099


These routines use the values r,c (row, column) to calcu-


late the actual memory address of the top left-hand corner


of the box and its corresponding attribute address. They


then take the width and height of the box and use the lower


of these two values as a loop counter within the machine


code to repeatedly call the appropriate routines until the


box is cleared. This saves using a For Next loop in Basic


and therefore speeds up the scrolling. However, this has a


slight disadvantage in that very small boxes are scrolled


off so fast that the effect is lost.


Because the machine code is not relocatable I have given


two decimal dumps


[ ...and this is where it goes wrong. The dumps given in


the article were hexdumps. The loader described was


absent; in its place was the usual Your Computer hex-


loader. There is talk of a demonstration program, which


is also missing. I've provided one of my own on the TZX,


which works on both 16K and 48K - it checks RAMTOP to


decide which code to load and which addresses to poke


and call.


Only a few of the sentences after this actually apply to


the code as published in the article; they are: ]


To load the code CLEAR 31231 for a 16K or 63999 for a 48K,


then LOAD ""CODE. [...] A good idea is to set up the


parameters of the box first, then load a screen$ from a


commercial game and finally run a routine to clear part of


the screen. This gives you a very interesting effect,


especially the diagonal scrolls.