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Extract Hex Number

Submitted by Steve Fewell

Routine: ExtrHex
Name: Extract Hex Number
Starting Address: &ADB7
Entry criteria: Text Pointer B points to the "&" character of the Hex Number. Y contains the Text Pointer B Offset.
Exit: IWA contains the extracted Integer number.

Clears the IWA to zero (using the FALSE routine - this also [importantly] sets X to zero) . Increment Y to point to the first character of the Hex number (after the "&").

Check the current Digit (&ADBB): 
Read the current character pointed to by Text Pointer B. If it is less than "0" then the Hex digit is invalid, so branch to the error routine (&ADE4). If the character is numeric ("0" to "9"), then jump to the process digit routine (&ADCF). If the character is more than "9" then Subtract 55 (so "A" becomes 10 and "F" becomes 15). If the new value is less than 10 or more than 15 then jump to the error routine (&ADE4) otherwise process the Hex digit. 

Process the Hex digit (&ADCF):
Multiply the number by 16 (This  moves all bits to the most significant half of the byte), this has two functions, firstly it removes the &30 (48) part of the ASCII value of the character, so that if the character is a digit between "0" and "9", it becomes it's numerical equivalent. And, secondary, it prepares the Hex digit so that the 4-bits specifying the digit's value can be pulled off the end.

Next a loop is repeated 4 times, in order to insert each bit of the 4-bit digit into the IWA. The loop moves the bits in the digit right 1 place (ASL), so that the carry contains the previous Most Significant Bit (and hence, the next bit of the digit). The Carry is rolled into the Least significant byte of the IWA, moving all of the existing bits left a position. Note: X equals -1 (&FF) after the loop has finished and the Hex digit has been processed.

Next, increment Y to the next character. If the end of the Text Line has not been reached (The character pointer equals 0 when this situation arises) then branch back to the Check Current Digit routine.

Error Routine (&ADE4):
Check X, if X = 0 then the Process Hex Digit routine has never been reached, i.e. The first character wasn't a valid Hex digit, so a "Bad Hex" error is generated. Otherwise if X is &FF (negative), then at least one digit has been extracted, so the fact that the next character is invalid signifies that the end of the Hex number has been reached, if this is the case then store Y back in &1B (the Text Pointer B Offset). Exit with A = #&40 (as an Integer number is being processed).

Note: No error checking is done to make sure that the Hex Number is not too large to fit in the IWA, instead the Most Significant bits of the IWA are moved out and lost if an overflow condition occurs (without the event being signified by an error message).


Disassembly for the Extract Hex Number routine

ADB7   032 232 171 20 E8 AB JSR &ABE8   FALSE
ADBA   200 C8 INY
ADBB   177 025 B1 19 LDA (&19),Y
ADBD 0 201 048 C9 30 CMP#&30
ADBF # 144 035 90 23 BCC 35 --> &ADE4
ADC1 : 201 058 C9 3A CMP#&3A
ADC3   144 010 90 0A BCC 10 --> &ADCF
ADC5 7 233 055 E9 37 SBC#&37
ADC7   201 010 C9 0A CMP#&0A
ADC9   144 025 90 19 BCC 25 --> &ADE4
ADCB   201 016 C9 10 CMP#&10
ADCD   176 021 B0 15 BCS 21 --> &ADE4
ADCF   010 0A ASL A
ADD0   010 0A ASL A
ADD1   010 0A ASL A
ADD2   010 0A ASL A
ADD3   162 003 A2 03 LDX#&03
ADD5   010 0A ASL A
ADD6 &* 038 042 26 2A ROL &2A
ADD8 &+ 038 043 26 2B ROL &2B
ADDA &, 038 044 26 2C ROL &2C
ADDC &- 038 045 26 2D ROL &2D
ADDF   016 244 10 F4 BPL -12 --> &ADD5
ADE1   200 C8 INY
ADE2   208 215 D0 D7 BNE -41 --> &ADBB
ADE4   138 8A TXA
ADE5   016 187 10 BB BPL -69 --> &ADA2  Bad Hex error
ADE7   132 027 84 1B STY &1B
ADE9 @ 169 064 A9 40 LDA#&40
ADEB ` 096 60 RTS


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