Detailed Instruction Reference

Note

  • Uppercase flags are affected by the instruction.
  • M refers to the value retrieved from memory, subject to the addressing mode.
  • .N where N is 0-7 refers to a single bit of a byte value. e.g. A.7 is bit 7 of the Accumulator register.
  • lower case letters are local variables e.g. t.
  • Since these docs are for a codebase that uses them, the actual CPU code may be more helpful than the pseudo-code presented here. The original logic is here in the name of preservation.

ADC

ADC Add Memory to A with Carry
Flags N V - b d i Z C
Address Mode Syntax OpCode I-Len T-Cnt
Immediate ADC #$A5 $69 2 2
Zero Page ADC $A5 $65 2 3
Zero Page,X ADC $A5,X $75 2 4
Absolute ADC $A5B6 $6D 3 4
Absolute,X ADC $A5B6,X $7D 3 4+
Absolute,Y ADC $A5B6,Y $79 3 4+
(Indirect,X) ADC ($A5,X) $61 2 6
(Indirect),Y ADC ($A5),Y $71 2 5+
+ Add 1 (one) T-State if a Page Boundary is crossed 
t = A + M + P.C
P.V = (A.7!=t.7) ? 1:0
P.N = A.7
P.Z = (t==0) ? 1:0
IF (P.D)
    t = bcd(A) + bcd(M) + P.C
    P.C = (t>99) ? 1:0
ELSE
    P.C = (t>255) ? 1:0
A = t & 0xFF
  • There is no add-without-carry instruction!
  • As the result of ADC depends on the contents of the Carry Flag (P.C)
  • When performing “single precision” (or “8 bit”) arithmetic it is often necessary to ensure that the Carry Flag (P.C) is CLEARed with a CLC command before the ADC is executed ...to ensure that the Carry Flag (P.C) has no bearing on the result.
  • CLEARing the Carry Flag (P.C) with a CLC command is normally necessary before the first stage of a “multiple precision” (or “more than 8 bit”) addition.
  • The action of ADC is dependant on the setting of Decimal Flag (P.D)

AND

AND Bitwise-AND A with Memory
Flags N v - b d i Z c
Address Mode Syntax OpCode I-Len T-Cnt
Immediate AND #$A5 $29 2 2
Zero Page AND $A5 $25 2 2
Zero Page,X AND $A5,X $35 2 3
Absolute AND $A5B6 $2D 3 4
Absolute,X AND $A5B6,X $3D 3 4+
Absolute,Y AND $A5B6,Y $39 3 4+
(Indirect,X) AND ($A5,X) $21 2 6
(Indirect),Y AND ($A5),Y $31 2 5+
+ Add 1 (one) T-State if a Page Boundary is crossed 
A = A & M
P.N = A.7
P.Z = (A==0) ? 1:0
AND 0 1   1 0 1 0 AND
0 0 0   1 1 0 0 =
1 0 1   1 0 0 0

ASL

ASL Arithmetic Shift Left
Flags N v - b d i Z C
Address Mode Syntax OpCode I-Len T-Cnt
Accumulator ASL A $0A 1 2
Zero Page ASL $A5 $06 2 5
Zero Page,X ASL $A5,X $16 2 6
Absolute ASL $A5B6 $0E 3 6
Absolute,X ASL $A5B6,X $1E 3 7 
P.C = B.7
B = (B << 1) & $FE
P.N = B.7
P.Z = (B==0) ? 1:0

P.C <- b.7 <- b.6 <- b.5 <- b.4 <- b.3 <- b.2 <- b.1 <- b.0 <- 0

Before: P.C = ? B = 1 1 1 0  1 1 1 0
After: P.C = 0 B = 1 1 0 1  1 1 0 0
  • In my experience, this is NOT an “arithmetic” shift
    • An Arithmetic shift normally preserves the Most Significant Bit (MSb) or “Sign bit” of the source value
    • ASL does NOT do this on the 6502.
    • The 6502 places a copy of the sign from the result of a Logical Shift Left into the sigN Flag (P.N)
  • This instruction would be better named as SLS (logical Shift Left and update Sign)

CLC