Bits and Representation · Lesson 22 of 30 · about 12 minutes
Bit Masks and Flags
Select individual bits with AND and set them with OR.
01 / Explain
Understand the idea
Eight bits can encode eight independent yes/no settings. In binary, 5 is 00000101, so AND 5 keeps bits 0 and 2 and clears the others. OR 2 sets bit 1 without clearing other bits.
Bitwise instructions use signed 32-bit values, even though memory holds eight-bit bytes. Our small nonnegative examples stay in the byte range. The zero flag tells you whether the selected bits were all clear.
02 / Try
Watch it happen
13 is binary 1101. Selecting bits 0 and 2 gives 5; setting bit 1 gives 7.
LOAD 13
AND 5
PRINT
OR 2
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read a byte from 0 through 255. Print only its bits 0 and 2 as a numeric mask, using AND 5.
The checker runs your current editor program in a fresh machine for each of 4 test cases. It supplies inputs and seeded memory itself; the lab’s current output and memory do not decide your result.
INPUT
# Keep bits 0 and 2.
PRINT
HALTNeed a hint?
AND keeps a bit only where both operands have a one.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
AND 5
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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