Adding Logo command parsing from a buffer in 6502 assembly requires handling string manipulation and tokenization. Below is an extended version of the previous code that includes a simple parser to read commands from a buffer and execute them.

### Assumptions
1. The input buffer contains a sequence of Logo commands separated by spaces or newlines.
2. Commands are in uppercase (e.g., `FORWARD 10`, `RIGHT 90`, `PENUP`).
3. The buffer is null-terminated.

### Updated 6502 Assembly Code

```assembly
; Constants
TURTLE_X     = $0200  ; Turtle X position
TURTLE_Y     = $0201  ; Turtle Y position
TURTLE_DIR   = $0202  ; Turtle direction (0=right, 1=down, 2=left, 3=up)
PEN_STATE    = $0203  ; Pen state (0=up, 1=down)
INPUT_BUFFER = $0300  ; Input buffer for commands
BUFFER_PTR   = $0204  ; Pointer to current position in input buffer

; Initialize turtle and buffer pointer
    LDA #$00
    STA TURTLE_X      ; Start at X=0
    STA TURTLE_Y      ; Start at Y=0
    STA TURTLE_DIR    ; Start facing right
    STA PEN_STATE     ; Start with pen down
    LDA #<INPUT_BUFFER
    STA BUFFER_PTR    ; Initialize buffer pointer to start of input buffer
    LDA #>INPUT_BUFFER
    STA BUFFER_PTR+1

; Main loop
MainLoop:
    JSR ReadCommand   ; Read a command
    JSR ExecuteCommand ; Execute the command
    JMP MainLoop      ; Repeat

; ReadCommand: Read a command from the input buffer
ReadCommand:
    LDY #$00          ; Reset Y for buffer indexing
    JSR SkipWhitespace ; Skip leading whitespace
    JSR ReadToken     ; Read the command token
    RTS

; SkipWhitespace: Skip spaces and newlines in the buffer
SkipWhitespace:
    LDA (BUFFER_PTR), Y
    CMP #' '          ; Space
    BEQ .NextChar
    CMP #$0A          ; Newline
    BEQ .NextChar
    RTS
.NextChar:
    INY
    JMP SkipWhitespace

; ReadToken: Read a token from the buffer
ReadToken:
    LDX #$00          ; X will index into the token buffer
.ReadChar:
    LDA (BUFFER_PTR), Y
    BEQ .EndOfBuffer  ; End of buffer
    CMP #' '          ; Space
    BEQ .EndOfToken
    CMP #$0A          ; Newline
    BEQ .EndOfToken
    STA TOKEN_BUFFER, X
    INX
    INY
    JMP .ReadChar
.EndOfToken:
    LDA #$00          ; Null-terminate the token
    STA TOKEN_BUFFER, X
    INY               ; Move past the space/newline
    RTS
.EndOfBuffer:
    LDA #$00          ; Null-terminate the token
    STA TOKEN_BUFFER, X
    RTS

; ExecuteCommand: Execute the command in TOKEN_BUFFER
ExecuteCommand:
    LDX #$00
.CompareCommand:
    LDA TOKEN_BUFFER, X
    CMP COMMAND_FORWARD, X
    BNE .NotForward
    INX
    CPX #7            ; Length of "FORWARD"
    BNE .CompareCommand
    JSR ParseNumber   ; Parse the number argument
    JSR MoveForward   ; Execute FORWARD command
    RTS
.NotForward:
    ; Repeat for other commands (RIGHT, LEFT, PENUP, PENDOWN)
    ; Example for RIGHT:
    LDX #$00
.CompareRight:
    LDA TOKEN_BUFFER, X
    CMP COMMAND_RIGHT, X
    BNE .NotRight
    INX
    CPX #5            ; Length of "RIGHT"
    BNE .CompareRight
    JSR ParseNumber   ; Parse the number argument
    JSR TurnRight     ; Execute RIGHT command
    RTS
.NotRight:
    ; Add similar blocks for LEFT, PENUP, PENDOWN
    RTS

; ParseNumber: Parse a number from the buffer
ParseNumber:
    LDY #$00
    LDA #$00
    STA NUMBER        ; Reset number
.ParseDigit:
    LDA (BUFFER_PTR), Y
    CMP #'0'
    BCC .EndParse     ; Not a digit
    CMP #'9'+1
    BCS .EndParse     ; Not a digit
    SEC
    SBC #'0'          ; Convert ASCII to number
    PHA               ; Save digit
    LDA NUMBER
    ASL A             ; Multiply by 10
    STA TEMP
    ASL A
    ASL A
    CLC
    ADC TEMP
    ADC #$00          ; Add new digit
    STA NUMBER
    PLA               ; Restore digit
    INY
    JMP .ParseDigit
.EndParse:
    RTS

; MoveForward: Move the turtle forward by n steps
MoveForward:
    LDX TURTLE_DIR
    LDA TURTLE_X
    CLC
    ADC ForwardDeltaX, X
    STA TURTLE_X
    LDA TURTLE_Y
    CLC
    ADC ForwardDeltaY, X
    STA TURTLE_Y
    RTS

ForwardDeltaX:
    .byte 1, 0, -1, 0  ; Delta X for each direction
ForwardDeltaY:
    .byte 0, 1, 0, -1  ; Delta Y for each direction

; TurnRight: Turn the turtle right by d degrees
TurnRight:
    LDA TURTLE_DIR
    CLC
    ADC #1
    AND #$03          ; Wrap around after 3
    STA TURTLE_DIR
    RTS

; TurnLeft: Turn the turtle left by d degrees
TurnLeft:
    LDA TURTLE_DIR
    SEC
    SBC #1
    AND #$03          ; Wrap around after 3
    STA TURTLE_DIR
    RTS

; PenUp: Lift the pen
PenUp:
    LDA #$00
    STA PEN_STATE
    RTS

; PenDown: Lower the pen
PenDown:
    LDA #$01
    STA PEN_STATE
    RTS

; Data
COMMAND_FORWARD:
    .byte "FORWARD", 0
COMMAND_RIGHT:
    .byte "RIGHT", 0
COMMAND_LEFT:
    .byte "LEFT", 0
COMMAND_PENUP:
    .byte "PENUP", 0
COMMAND_PENDOWN:
    .byte "PENDOWN", 0

TOKEN_BUFFER:
    .res 16           ; Reserve 16 bytes for token buffer
NUMBER:
    .byte 0           ; Storage for parsed number
TEMP:
    .byte 0           ; Temporary storage
```

### Explanation of Changes

1. **Input Buffer**: The `INPUT_BUFFER` holds the Logo commands. The `BUFFER_PTR` keeps track of the current position in the buffer.
2. **Tokenization**: The `ReadToken` routine reads a command or argument from the buffer into `TOKEN_BUFFER`.
3. **Command Parsing**: The `ExecuteCommand` routine compares the token with known commands (`FORWARD`, `RIGHT`, etc.) and calls the appropriate subroutine.
4. **Number Parsing**: The `ParseNumber` routine converts ASCII digits in the buffer to a binary number.
5. **Command Execution**: The `MoveForward`, `TurnRight`, `TurnLeft`, `PenUp`, and `PenDown` routines implement the Logo commands.

### Example Input Buffer
If the input buffer contains:
```
FORWARD 10 RIGHT 90 FORWARD 20 PENUP
```
The program will parse and execute these commands sequentially.

### Limitations
- This implementation assumes fixed-format input (e.g., no lowercase commands).
- Error handling is minimal; invalid commands or numbers will cause undefined behavior.
- The buffer size and token buffer size are fixed and may need adjustment for larger inputs.

This code provides a basic framework for parsing and executing Logo commands in 6502 assembly. You can expand it further to handle more commands, error checking, and graphics output.