By John Hardman, Andrew D. Hewson
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Extra resources for 40 Best Machine Code Routines for the ZX Spectrum
At `subtract', if (c-1) is a multiple of 64, 1792 is subtracted from de, so that de points to the next block of 64 lines. If c does not hold one, the routine jumps to `next line', otherwise the routine returns to BASIC. 56 6. DISPLAY ROUTINES Merge Pictures Length: 21 Number of Variables: 1 Check sum: 1709 Operation This routine merges a picture stored in RAM (using the `Copy' routine elsewhere in this book) with the current screen display. The attributes are not changed. Variables Name Length Location screen store Comment 2 23296 address in RAM of stored picture Call RAND USR address Error Checks None Comments To merge pictures the routine should be used as listed.
Consider any bit B 2 of byte B, in Figure B1. The data held at the location (B 2 , B,) in the matrix will be Each byte of the rotated character will be built up one at a time, by adding the values of all the bits N 2 that will be in the new byte. The hl register is loaded with the address of the first byte of the character in RAM. The e register is loaded with the value of the byte which has bit 7 on and bits 0-6 off ie 128. The hl register is saved on the stack. The c register, to which data will be added giving the new value of the byte being built, is loaded with zero.
The squares represent illuminated pixels, x marks the starting position within the area to be shaded and * marks the rightmost pixels of RUNS. FA j j j j j j ^ 0 j 0 j 7 ^ j j rzz j Ô j j A j * * * * j A A * * * A Figure B2. An illustration of the techn ique used for filling a region. Grey squares are already illuminated and define the region to be shaded. X is the starting position, * are the starts of RUNS and O remain unshaded. How it works This routine plots horizontal lines of adjacent pixels called `RUNS' within areas bounded by illuminated pixels.
40 Best Machine Code Routines for the ZX Spectrum by John Hardman, Andrew D. Hewson