cpct_drawStringM2

Prints a null-terminated string with ROM characters on a given byte-aligned position on the screen in Mode 2 (640x200 px, 2 colours).

C Definition

void cpct_drawStringM2 (void* string, void* video_memory, u8 pen)

Input Parameters (4 Bytes)

Assembly call (Input parameters on registers)

call cpct_drawStringM2_asm

Parameter Restrictions

• string must be a null-terminated string.  It could contain any 8-bit value as characters except 0, which will signal the end of the string.  Be careful to provide strings with a 0 (null) at the end of the string.  Otherwise, unexpected results may happen (Typically, rubbish characters printed on screen and, occasionally, memory overwrite and even hangs or crashes).
• video_memory could theoretically be any 16-bit memory location.  It will work outside current screen memory boundaries, which is useful if you use any kind of double buffer.  However, be careful where you use it, as it does no kind of check or clipping, and it could overwrite data if you select a wrong place to draw.
• pen 0 = Inverted, >0 = Normal.  Normal means foreground colour = PEN 1, background colour = PEN 0.  Inverted means the contrary of normal.

Requirements and limitations

• Do not put this function’s code below 0x4000 in memory.  In order to read characters from ROM, this function enables Lower ROM (which is located 0x0000-0x3FFF), so CPU would read code from ROM instead of RAM in first bank, effectively shadowing this piece of code.  This would lead to undefined results (typically program would hang or crash).
• This routine does not check for boundaries.  If you draw too long strings or out of the screen, unpredictable results will happen.
• Screen must be configured in Mode 2 (640x200 px, 2 colours)
• This function requires the CPC firmware to be DISABLED.  Otherwise, random crashes might happen due to side effects.
• This function disables interrupts during main loop (character printing), and re-enables them at the end.
• This function will not work from ROM, as it uses self-modifying code.

Details

This function receives a null-terminated string and draws it to the screen in Mode 2 (640x200, 2 colours).  This function calls cpct_drawCharM2 to draw every character.  video_memory parameter points to the byte where the string will be drawn.  The first pixel of that byte will be the upper-left corner of the string.  As this function uses a byte-pointer to refer to the upper-left corner of the string, it can only draw string on module-8-pixel columns (0, 8, 16, 24...), as every byte contains 8 pixels in Mode 2.

Destroyed Register values

AF, BC, DE, HL

Required memory

109 bytes (33 bytes this function, 76 bytes cpct_drawCharM2)

Time Measures

  Case     |   Cycles    |   microSecs (us)
-------------------------------------------
  Best     | 143 + 799*L | 28.75 + 199.75*L
  Worst    | 143 + 843*L | 28.75 + 210.75*L
----------------------------------------------
Asm saving |       -84   |         -21.00
----------------------------------------------

L = Length of the string (excluding null-terminator character)

These time measures take into account the time it takes to draw each individual character (call to cpct_drawCharM2, assembly entry point).