Micro­power

and the number of characters in the string in NUMN.

SCAL CRT, DF 65

This outputs the contents of the accumulator to the screen. The routine first tests for a null (00) or a line feed (0AH); these are ignored. The next control code to be handled is Clear Screen (0CH). On receipt of this code 48 spaces are written in the top line (£080A – £0839), 16 nulls are written in the margin (£83A – £849), the line is copied 16 times to fill the whole screen, and the cursor is repositioned to the top left.

The remaining control codes are tested for in turn and the appropriate action taken. Normal characters are inserted at the current cursor position and the cursor moved one space right. If this takes it into the screen margin, the margin bytes are skipped. When the cursor moves beyond the limits of the screen RAM, the screen is scrolled by copying the bottom 14 screen lines (£084A – £0BB9) up one line, clearing the bottom line, and repositioning the cursor to the bottom left.

SCAL TBCD3, DF 66

The contents of the HL register pair are printed as a four-character hexadecimal number followed by a space by this routine. The H register is first transferred to the accumulator and printed out using SCAL TBCD2, DF 67; the L register is then treated similarly, and a space character is then output.

SCAL TBCD2, DF 67

This routine prints out the contents of the accumulator as two hexadecimal digits; it differs from SCAL B2HEX, DF 68, which forms the last part of the subroutine, only in that it adds the byte being printed to the checksum in register C.

SCAL B2HEX, DF 68

The contents of the accumulator are rotated right four times so that the most significant nibble becomes the least significant nibble. SCAL B1HEX, DF 7A, is then used to print the necessary hex digit. The accumulator contents are recovered, and the appropriate digit for the bottom nibble is printed. The method of converting each nibble is rather clever; 90H is added to the nibble and the DAA (Decimal Adjust Accumulator) instruction is applied to the result. This produces a value in the range 90H – 99H, with the carry flag reset if the original nibble is in the range 0 – 9. When 40H is added with the carry flag, the result lies in the range 30H – 39H, or 41H – 46H; i.e., the appropriate hex digit has been produced!