0218-PRINT-DIGITS.md

0218 Print digits

SYS0218: PUSH AX
SYS0219: MOV AL,AH
SYS021B: CALL 021F
SYS021E: POP AX

This will convert number in AX to ASCII. It first converts the upper half (8-bits) in AH by a call to SYS:021F (see below), copying AH into AL beforehand. When it returns, it will now proceed to convert the bottom half in AL.

SYS021F: PUSH AX
SYS0220: MOV CL,04
SYS0222: SHR AL,CL
SYS0224: CALL 022A
SYS0227: POP AX

This will convert the number in AL to an ASCII character. It does the conversion one nibble (4-bits) at a time, converting the upper 4-bits first then calling SYS:022A (see below). The upper 4-bits are shifted right into the lower 4-bits prior to the call.

SYS0228: AND AL,0F
SYS022A: ADD AL,30
SYS022C: CMP AL,3A
SYS022E: JB 0232

Clears upper half (or the upper 4-bits) of AL then converts AL to an ASCII character. If the ASCII character is from '0' to '9' then go ahead and print it by jumping to SYS:0232 Print Character.

Why is the upper half cleared first? This ensures that the conversion is handled properly and the output limited to '0'-'9' and 'A' to 'F'. If this the call was made to SYS:022A instead of SYS:0228, it is assumed that the upper half is already cleared, usually by shifting AL to the right by 4 bits (see above).

SYS0230: ADD AL,07

This convert ASCII digits above '9' to 'A' - 'F'.

From here, it continues into the SYS:232 Print Character, to print the ASCII character in AL.

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