| &BBFF | SCR INITIALISE |
| Action | Initialises the Screen Pack to the default values used when the computer is first switched on |
| Entry | No entry conditions |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| Notes | All screen indirections are restored to their default settings, as are inks and flashing speeds; the mode is switched to MODE 1 and the screen is cleared with PEN 0; the screen address is moved to &C000 and the screen offset is set to zero |
| &BC02 | SCR RESET |
| Action | Resets the Screen Pack's indirections, flashing speeds and inks to their default values |
| Entry | No entry conditions |
| Exit | AF, BC, DE r1nd HL are corrupt, and all other registers are preserved |
| &BC05 | SCR SET OFFSET |
| Action | Sets the screen offset to the specified values - this can cause the screen to scroll |
| Entry | HL contains the required offset, which should be even |
| Exit | AF and HL are corrupt, and alI others are preserved |
| Notes | The screen offset is reset to 0 whenever its mode is set, or it is cleared by SCR CLEAR (but not BASIC's CLS) |
| &BC08 | SCR SET BASE |
| Action | Sets the location in memory of the screen - effectively can only be &C000 or &4000 |
| Entry | A contains the most significant byte of the screen address required |
| Exit | AF and HL are corrupt, and all other registers are preserved |
| Notes | The screen memory can only be set at 16K intervals (ie &0000, &4000, &8000, &C000) and when the computer is first switched on the 16K of screen memory is located at &C000) |
| &BC0B | SCR GET LOCATION |
| Action | Gets the location of the screen memory and also the screen offset |
| Entry | No entry conditions |
| Exit | A holds the most significant byte of the screen address, HL holds the current offset, and all others are preserved |
| &BC0E | SCR SET MODE |
| Action | Sets the screen mode |
| Entry | A contains the mode number - it has the same value and characteristics as in BASIC |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| Notes | The windows are set to cover the whole screen and the graphics origin is set to the bottom left corner of the screen; in addition, the current stream is set to zero, and the screen offset is zeroed |
| &BC11 | SCR GET MODE |
| Action | Gets the current screen mode |
| Ently | No entry conditions |
| Exit | If the mode is 0, then Carry is true, Zero is false, and A contains 0; if the mode is 1, then Carry is false, Zero is true, and A contains 1; if the mode is 2, then Carry is false, Zero is false, and A contains 2; in all cases the other flags are corrupt and all the other registers are preserved |
| &BC14 | SCR CLEAR |
| Action | Clears the whole of the screen |
| Entry | No entry conditions |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| &BC17 | SCR CHAR LIMITS |
| Action | Gets the size of the whole screen in terms of the numbers of characters that can be displayed |
| Entry | No entry conditions |
| Exit | B contains the number of characters across the screen, C contains the number of characters down the screen, AF is corrupt, and all other registers are preserved |
| &BC1A | SCR CHAR POSITION |
| Action | Gets the memory address of the top left corner of a specified character position |
| Entry | H contains the character physical column and L contains the character physical row |
| Exit | HL contains the memory address of the top left comer of the character, B holds the width in bytes of a character in the present mode, AF is corrupt, and all other registers are preserved |
| &BC1D | SCR DOT POSITION |
| Action | Gets the memory address of a pixel at a specified screen position |
| Entry | DE contains the base X-coordinate of the pixel, and HL contains the base Y-coordinate |
| Exit | HL contains the memory address of the pixel, C contains the bit mask for this pixel, B contains the number of pixels stored in a byte minus 1, AF and DE are corrupt, and all others are preserved |
| &BC20 | SCR NEXT BYTE |
| Action | Calculates the screen address of the byte to the right of the specified screen address (may be on the next line) |
| Entry | HL contains the screen address |
| Exit | HL holds the screen address of the byte to the right of the original screen address, AF is corrupt, all others are preserved |
| &BC23 | SCR PREV BYTE |
| Action | Calculates the screen address of the byte to the left of the specified screen address (this address may actually be on the previous line) |
| Entry | HL contains the screen address |
| Exit | HL holds the screen address of the byte to the left of the original address, AF is corrupt, all others are preserved |
| &BC26 | SCR NEXT LINE |
| Action | Calculates the screen address of the byte below the specified screen address |
| Entry | HL contains the screen address |
| Exit | HL contains the screen address of the byte below the original screen address, AF is corrupt, and all the other registers are preserved |
| &BC29 | SCR PREV LINE |
| Action | Calculates the screen address of the byte above the specified screen address |
| Entry | HL contains the screen address |
| Exit | HL holds the screen address of the byte above the original address, AF is corrupt, and all others are preserved |
| &BC2C | SCR INK ENCODE |
| Action | Converts a PEN to provide a mask which, if applied to a screen byte, will convert all of the pixels in the byte to the appropriate PEN |
| Entry | A contains a PEN number |
| Exit | A contains the encoded value of the PEN, the flags are corrupt, and all other registers are preserved |
| Notes | The mask returned is different in each of the screen modes |
| &BC2F | SCR INK DECODE |
| Action | Converts a PEN mask into the PEN number (see SCR INK ENCODE for the re~ erse process) |
| Entry | A contains the encoded value of the PEN |
| Exit | A contains the PEN number, the flags are corrupt, and all others are preserved |
| &BC32 | SCR SET INK |
| Action | Sets the colours of a PEN - if the two values supplied are different then the colours will alternate (flash) |
| Entry | contains the PEN number, B contains the first colour, and C holds the second colour |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| &BC35 | SCR GET INK |
| Action | Gets the colours of a PEN |
| Entry | A contains the PEN nurnber |
| Exit | B contains the first colour, C holds the second colour, and AF, DE and HL are corrupt, and all others are preserved |
| &BC38 | SCR SET BORDER |
| Action | Sets the colours of the border - again if two different values are supplied, the border will flash |
| Entry | B contains the first colour, and C contains the second colour |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| &BC3B | SCR GET BORDER |
| Action | Gets the colours of the border |
| Entry | No entry conditions |
| Exit | B contains the first colour, C holds the second colour, and AF, DE and HL are corrupt, and all others are preserved |
| &BC3E | SCR SET FLASHING |
| Action | Sets the speed with which the border's and PENs' colours flash |
| Entry | H holds the time that the first colour is displayed, L holds the time the second colour is displayed for |
| Exit | AF and HL are corrupt, and all other registers are preserved |
| Notes | The length of time that each colour is shown is measured in 1/5Oths of a second, and a value of 0 is taken to mean 256 * 1/50 seconds - the default value is 10 * 1/50 seconds |
| &BC41 | SCR GET FLASHING |
| Action | Gets the periods with which the colours of the border and PENs flash |
| Entry | No entry conditions |
| Exit | H holds the duration of the first colour, L holds the duration of the second colour, AF is corrupt, and all other registers are preserved - see SCR SET FLASHING for the units of time used |
| &BC44 | SCR FILL BOX |
| Action | Fills an area of the screen with an ink - this only works for `character-sized' blocks of screen |
| Entry | A contains the mask for the ink that is to be used, H contains the left hand colurnn of the area to fill, D contains the right hand column, L holds the top line, and E holds the bottom line of the area (using physical coordinates) |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| &BC17 | SCR FLOOD BOX |
| Action | Fills an area of the screen with an ink - this only works for `byte-sized' blocks of screen |
| Entry | C contains the encoded PEN that is to be used, HL contains the screen address of the top left hand corner of the area to fill, D contains the width of the area to be filled in bytes, and E contains the height of the area to be filled in screen lines |
| Exit | AF, BC, DE and HL are corrupt, and all other registers are preserved |
| Notes | The whole of the area to be filled must lie on the screen otherwise unpredictable results may occur |
| &BC4A | SCR CHAR INVERT |
| Action | Inverts a character's colours; all pixels in one PEN's colour are printed in another PEN's colour, and vice versa |
| Entry | B contains one encoded PEN, C contains the other encoded PEN, H contains the physical column number, and L contains the physical line number of the character that is to be inverted |
| Exit | AF, BC, DE and HL are corrupt, and alI the other registers are preserved |
| &BC4D | SCR HW ROLL |
| Action | Scrolls the entire screen up or down by eight pixel rows (ie one character line) |
| Entry | B holds the direction that the screen will roll, A holds the encoded PAPER which the new line will appear in |
| Exit | AF, BC, DE and HL are corrupt, and all others are preserved |
| Notes | This alters the screen offset; to roll down, B must hold zero, and to roll upwards B must be non-zero |
| &BCS0 | SCR SW ROLL |
| Action | Scrolls part of the screen up or down by eight pixel lines - only for `character-sized' blocks of the screen |
| Entry | B holds the direction to roll the screen, A holds the encoded PAPER which the new line will appear in, H holds the left column of the area to scroll, D holds the right colurnn, L holds the top line, E holds the bottom line |
| Exit | AF, BC, DE and HL are corrupt, and all other registers are preserved |
| Notes | The area of the screen is moved by copying it; to roll down, B must hold zero, and to roll upwards B must be non-zero; this routine uses physical roordinates |
| &BC53 | SCR UNPACK |
| Action | Changes a character matrix from its eight byte standard form into a set of pixel masks which are suitable for the current mode - four *8 bytes are needed in mode 0, two *8 bytes in mode l, and 8 bytes in mode 2 |
| Entry | HL contains the address of the matrix, and DE contains the address where the masks are to be stored |
| Exit | AF. BC, DE and HL are corrupt, and all other registers are preserved |
| &BC56 | SCR REPACK |
| Action | Changes a set of pixel masks (for the current mode) into a standard eight byte character matrix |
| Entry | A contains the encoded foreground PEN to be matched against (ie the PEN that is to be regarded as being set in the character), H holds the physical column of the character to be `repacked', L holds the physical line of the character, and DE contains the address of the area where the character matrix will be built |
| Exit | AF, BC, DE amd HL are corrupt, and all the others are preserved |
| &BC59 | SCR ACCESS |
| Action | Sets the screen write mode for graphics |
| Entry | A contains the write mode (0=Fill, 1=XOR, 2=AND, 3=OR) |
| Exit | AF. BC, DE and HL are corrupt, amd all other registers are preserved |
| Notes | The fill mode means that the ink that plotting was requested in is the ink that appears on the screen; in XOR mode, the specified ink is XORed with ink that is at that point on the screen already before plotting; a simiIar situation occurs with the AND and OR modes |
| &BC5C | SCR PIXELS |
| Action | Puts a pixel or pixels on the screen regardless of the write mode specified by SCR ACCESS above |
| Entry | B contains the mask of the PEN to be drawn with, C contains the pixel mask, and HL holds the screen address of the pixel |
| Exit | AF is corrupt, amd all others are preserved |
| &BC5F | SCR HORIZONTAL |
| Action | Draws a honzontal line on the screen using the current graphics write mode |
| Entry | A contains the encoded PEN to be drawn with, DE contains the base X-coordinate of the start of the line, BC contains the end base X-coordinate, and HL contains the base Y-coordinate |
| Exit | AF, BC, DE and HL are conupt, and all other registers are preserved |
| Notes | The start X-coordinate must be less than the end X- coordmate |
| &BC62 | SCR VERTICAL |
| Action | Draws a vertical line on the screen using the current graphics write mode |
| Entry | A contains the encoded PEN to be drawn with, DE contains the base X-coordinate of the line, HL holds the start base Y-coordinate, and BC contains the end base Y-coordinate - the start coordinate must be less than the end coordinate |
| Exit | AF, BC, DE and HL are corrupt, and all the other registers are preserved
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