The following example calculates the address of an attribute at character position (b,c) and returns it in the HL register pair.

By then examining the contents of this address we can find out the colours displayed at a particular position, and act accordingly. To find the address of a colour cell at print position (x,y) we therefore need only to multiply x by 32, add y, then add 22528 to the result. In other words, the screen’s top 32 attribute cells are located at addresses 22528 to 22559 going left to right, the second row of colour cells from 22560 to 22591 and so on.

Unlike the Spectrum’s awkward pixel layout, colour cells, located at addresses 22528 to 23295 inclusive, are arranged sequentially in RAM as one would expect. It is also useful to calculate an attribute’s address, and not just its value, in case we want to write to it as well. However, it is much faster to do the calculation ourselves. The simplest way to find an attribute value is to use a couple of ROM routines: ld bc,(ballx) put x and y in bc register pair.Ĭall 9603 call ROM to put attribute (c,b) on stack.Ĭall 11733 put attributes in accumulator. A quick look through the Spectrum’s ROM disassembly reveals a routine at address 9603 which will do the job for us, or we can calculate the memory address ourselves. There are two ways to find the colour attribute settings for a particular character cell on the Spectrum. The method was so useful in fact that it its machine language equivalent was employed by a number of commercial games, and it is of great use to the novice Spectrum programmer. This was a way to detect the colour attributes of any particular character cell on the screen, and though tricky for the BASIC programmer to grasp, could be very handy for simple collision detection. Note: This article was originally written by Jonathan Cauldwell and is reproduced here with permission.Īnyone who ever spent time programming in Sinclair BASIC may well remember the ATTR function.

Sinclair zx spectrum symbos