Byte Memory · Lesson 9 of 30 · about 9 minutes
Bytes and Boundaries
See why memory wraps while registers do not.
01 / Explain
Understand the idea
A byte has 256 possible values, 0 through 255. STORE keeps the low eight bits of a number: 256 becomes 0, −1 becomes 255, and 511 becomes 255. This is the documented value & 255 rule.
Registers keep safe integer arithmetic. LOAD 256 still places 256 in R0; only storing it into byte memory changes its representation. The overflow flag reports a signed 32-bit boundary without automatically wrapping arithmetic.
02 / Try
Watch it happen
Expect 256, 0, 255. Address 255 is valid; address 256 is not.
LOAD 256
PRINT
STORE 255
LOAD [255]
PRINT
LOAD -1
STORE 255
LOAD [255]
PRINT
HALTUse Step to follow one instruction at a time. You can change the example and replay it.
03 / Challenge
Make it work
Read an integer, store it at address 255, retrieve the resulting byte, and print it. Do not calculate a substitute fixed answer.
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
# Store at the final valid address, then read back.
HALTNeed a hint?
The CPU’s STORE instruction already performs byte conversion.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
STORE 255
LOAD [255]
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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