Byte Memory · Lesson 8 of 30 · about 9 minutes
Addresses in Registers
Use an address as data and read a lookup table.
01 / Explain
Understand the idea
A register can hold an address. LOAD [R1] reads memory at the address currently stored in R1; LOAD R1 reads R1 itself. This distinction lets one instruction work with many memory cells.
Addresses must be whole integers from 0 to 255. A value outside that range is an error, not an invitation to wrap around. Brackets identify a memory read only for LOAD; STORE R1 uses the register value as its destination address.
02 / Try
Watch it happen
R1 holds 7, while R0 receives the byte 99 from cell 7.
STORE 12 0
STORE 99 7
LOAD 7 R1
LOAD [R1]
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 address from INPUT, copy it to R1, retrieve the seeded byte at that address, and print the byte. The checker changes both address and stored 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
COPY R0 R1
# Read through R1.
HALTNeed a hint?
The brackets belong around R1, not around the input instruction.
Reveal a worked solution
Read the program, predict each instruction’s effect, then step through it in the lab.
INPUT
COPY R0 R1
LOAD [R1]
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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