on 3/4/04 1:28 PM, KirkIsrael@xxxxxxxxxxxxx at KirkIsrael@xxxxxxxxxxxxx
wrote:
>> Hi there,
>>
>>> I think what you want is:
>>> LDA var
>>> CMP #MAX
>>> BMI ltMAX
>>> LDA #MAX
>>> ltMAX
>>> CMP #MIN
>>> BPL gtMIN
>>> LDA #MIN
>>> gtMIN
>>> STA var
Or you can cheat...
Since you're comparing a signed number to a constant... Do this...
LDA Value
EOR #$80
CMP #CONSTANT+$80
BCC LESS
BCS GREATEREQUAL
By adding 0x80 to the number, you convert 0-0x7F to 0x80-0xFF and 0x80-0xFF
become 0x00-0x7F. Now, this turns your signed range into an UNSIGNED range.
By the performing the UNSIGNED CMP instruction, you can use BCC and BCS with
100% accuracy.
You see, there is a problem with overflow for signed numbers in 6502. To get
around it, there is the overflow flag. The proper way to do signed compares
was...
LDA Value
CMP #$80
BVC NoOverflow
BMI GreaterEqual
BPL Less
NoOverflow
BPL GreaterEqual
BMI Less
That code was a joke.
Now, if you can guarantee that there is no overflow (Both values are like
sign and the subtraction won't cross the $80 boundary, then you can ditch
the BVC (Normal case) and just use BPL for Greaterequ and BMI for less (Most
people do this anyways.
I use the EOR #$80 trick for many years and it's the fastest way to compare
a value to a constant with no fancy stuff.
Burger
----------------------------------------------------------------------------------------------
Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/
