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01FE-CONVERT-NUMBER.md

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01FE Convert number to ASCII

This subroutine takes an unsigned number in AL to a string. The base of the string is set in CL.

SYS01FE: MOV CL,64
SYS0200: CALL 020A

This converts the number in AL into a base-100 digit. This takles only the upper digit in AL and converts it to '0', '1', or '2'.

SYS0203: MOV CL,0A
SYS0205: CALL 020A
SYS0208: JMP 020E

This converts the number in AL into a base-10 string. This works best only if AL is less than 100 (64h). It converts the first half with a call to SYS:020A, then converts the second half with a jump to SYS:020E (see below).

SYS020A: XOR AH,AH
SYS020C: DIV CL

This is the actual digit to ascii conversion. It is possible to call this subroutine at offset SYS:020A. To do so, ensure that CL contains the base (1-255/01h-FFh) and AL contains the number to convert (0-255/00h-FFh).

Conversion is done one digit at a time. First, it clears the upper half of AX, i.e. AH and divides the AX by the base in CL. The result is in AL while the remainder is in AH.

SYS020E: ADD AL,30
SYS0210: PUSH AX
SYS0211: CALL 0232
SYS0214: POP AX

This converts the result in AL to ASCII by adding 30h ('0') and calling SYS:0232 Print Digits to print it.

SYS0215: MOV AL,AH
SYS0217: RET

This puts the remainder back in AL then returns to the calling subroutine. If this was called from SYS:0200, it would have converted the upper digit already and will now proceed to SYS:0203 and convert the lower digits. Otherwise, it will return to the caller. Because it returns with a NEAR RET, it can only be called from within the system library.

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