Bits and Representation · Lesson 23 of 30 · about 12 minutes
Toggling Bits
Use XOR’s reversible change to update a saved setting.
01 / Explain
Understand the idea
XOR flips a bit when the mask bit is one and preserves it when the mask bit is zero. XOR 6 toggles bits 1 and 2. Applying the same mask twice restores the original value.
A reversible operation can be tested with an invariant: after two toggles, the saved setting equals the starting setting. Memory makes this visible even when R0 has changed during the program.
02 / Try
Watch it happen
Expect 44 then 42. Compare the changed and restored bits.
LOAD 42
XOR 6
PRINT
XOR 6
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read one byte. Toggle bits 1 and 2, store the toggled byte at address 20, and print it after reading it back. Toggle those bits again, save the restored byte at address 20, and print the restored value.
The checker runs your current editor program in a fresh machine for each of 3 test cases. It supplies inputs and seeded memory itself; the lab’s current output and memory do not decide your result.
INPUT
XOR 6
STORE 20
LOAD [20]
PRINT
# Toggle again, save and print the restored value.
HALTNeed a hint?
Use XOR 6 both times; the final memory byte must equal the original input.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
XOR 6
STORE 20
LOAD [20]
PRINT
XOR 6
STORE 20
LOAD [20]
PRINT
HALTEmoji CPU lab
Emoji program
Type LOAD, ADD, or another opcode then Space to insert emoji. Ctrl/⌘ + Enter runs or pauses; Escape pauses; Ctrl/⌘ + ] indents. Tab moves focus. Labels use a colon. Jumps use zero-based instruction addresses.
Instruction map and breakpoints (0)
Breakpoints stop before an instruction. Run resumes past the stopped breakpoint once; Step executes it directly. Editing source clears old breakpoints and machine state.
CPU registers
- R0
- 0
- R1
- 0
- R2
- 0
- R3
- 0
- R4
- 0
- R5
- 0
- R6
- 0
- R7
- 0
Stacks and loop frames
SP = 255 − data depth − call depth. The stack is separate from memory.
Data stack (bottom → top)
Empty
Call return addresses (bottom → top)
Empty
Loop frames
Empty
Output and input
Run a PRINT instruction to see output.
Queued input: Empty
Memory · 256 bytes · 0 nonzero
Each cell shows address:value. R = read this step; W = written this step. Select a cell to inspect or initialize it before execution. Use arrow keys to move, Home/End for the row, and Ctrl/⌘ + Home/End for the whole memory.
Execution trace · 0 entries
Recent entries below. Inspect any zero-based index to see complete detached before/after state.
Check your challenge
You can run this check any time. Every case must pass to record completion.
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