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--- aslfaqs [2025/09/23 17:28] – [How do negative numbers work?] reggie
+++ aslfaqs [2025/09/23 17:33] (current) – [How do negative numbers work?] reggie
@@ Line 659: / Line 659: @@
 In binary the principle works just the same, only (for an 8-bit value) we subtract from 256 instead of 60, ie. "minus ten" equals "256-10", being 246, or in hex $f6. Then if we see 246 and want to see the "minus" variant, we do the same, subtract it from 256.
-Which signed numbers are positive and which negative? Highest bit set means negative, so unsigned values 128 to 255 represent -128 to -1. Positive signed values range from 0 to 127. This is called "two's-complement" format, beacuse to negate we can complement (COM, or flip) all the bits and add 1 to the result.
+Which signed numbers are positive and which negative? Highest bit set means negative, so unsigned values 128 to 255 represent -128 to -1. Positive signed values range from 0 to 127. This is called "two's-complement" format, because to negate we can flip (or COMplement) all the bits and add 1 to the result.
 This format is chosen because the underlying circuitry doesn't need to change; it's simply down to the programmer to keep track of which variables are seen as signed and unsigned. There are two flags in the condition codes to help with signed numbers: the N flag is set whenever bit 7 of a result is set, and V is set when a signed result overflows. An overflow is an unintended sign change; to use our clock analogy it's when the minute hand passes through the half-hour without going through the hour first.
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 Which instructions deal with signed numbers? There's SEX (Sign EXtend) which changes a signed value in B to a signed value in D by simply filling A with 8 copies of bit 7 of B. To negate an 8-bit value use NEG. All the addition and subtraction instructions work with either interpretation, but note that MUL is always unsigned. The conditional branches (also having long variants) are:
-  Signed branches   Meaning             Unsigned equivalent
+  Signed branches    Meaning             Unsigned equivalent
-       BPL             Plus
+       BPL             PLus
-       BMI             Minus
+       BMI             MInus
-       BLT             Less-than                BLO
+       BLT             Less-Than                BLO
-       BLE             Less-than or equal       BLS
+       BLE             Less-than or Equal       BLS
-       BGT             Greater-than             BHI
+       BGT             Greater-Than             BHI
-       BGE             Greater-than or equal    BHS
+       BGE             Greater-than or Equal    BHS
   Miscellaneous instructions
-  NEG <accumulator or memory> ( NEGate, 8-bit only)
+  NEG (8-bit only)     NEGate                   COM
-  SEX <inherent>              (Sign EXtend B to D)
+  SEX                  Sign EXtend B to D
-  COMA; COMB; ADDD #1         (negate D)
+  COMA; COMB; ADDD #1  negate D
 Note that other than the simple BPL/BMI, the signed variants are not commonly used. If you do choose one, think twice whether you meant the unsigned variant. Signed comparisons should not be used with addresses for example.