diff --git a/Programs/CosmOS/README.md b/Programs/CosmOS/README.md
index 0fc426f..ec1aef7 100644
--- a/Programs/CosmOS/README.md
+++ b/Programs/CosmOS/README.md
@@ -776,6 +776,33 @@ cent, where without it the same change costs sixty eight - which is the point: *
 that reads ahead stops caring very much how fast its disk is.** The three per cent it costs
 at zero is the bookkeeping, paid when there is nothing to hide behind it.
 
+### And Waiting For What Is Left:
+
+Read ahead hides most of the wait and cannot hide all of it. What remained was a loop
+asking the disk's status port over and over, which is work the machine is doing and memory
+it is touching to find out that nothing has happened yet.
+
+`sbfsWaitDisk` uses `WAIT` now. It tests the status port, and only if the disk is still
+busy does it stop - the CPU is put down until a device raises a line, and the disk raises
+one when it finishes. **Asking first is what makes it safe:** if the disk finished in the
+gap between the test and the `WAIT`, its line is already standing and the `WAIT` does
+nothing rather than sleeping through the answer.
+
+There is no handler and no vector. The shell keeps the Interrupt Flag down, and a `WAIT`
+wakes on a line whether or not anybody intends to answer it, taking it down on the way
+past. Printing the same fourteen kilobyte file:
+
+| Cycles a block | Total | Of that, the bus | Waiting |
+| -- | -- | -- | -- |
+| 0 | 922,570 | 922,570 | 0 |
+| 2,000 | 946,474 | 922,702 | 23,772 |
+| 10,000 | 1,042,474 | 922,702 | 119,772 |
+
+**The middle column stops moving.** What the program costs in memory is now the same
+whatever the disk does, and the difference is time spent with the bus quiet. On this
+emulator that changes nothing anybody can see; on hardware it is the difference between a
+CPU contending for memory with everything else and a CPU standing out of the way.
+
 ### Saving Something Twice:
 
 Which is why saving a document is not the same as writing a file, and why sbfsSaveFile exists rather than each tool doing it. A file that has grown will usually not fit where it was, so saving it means putting it somewhere else and letting go of where it was - and **the obvious order is a trap**:
diff --git a/Programs/CosmOS/Source/sbfs.asm b/Programs/CosmOS/Source/sbfs.asm
index 30f6ffe..0044e5d 100644
--- a/Programs/CosmOS/Source/sbfs.asm
+++ b/Programs/CosmOS/Source/sbfs.asm
@@ -1892,12 +1892,23 @@ sbfsForgetBuffer:
 ; settled status, and CALL would put A back the way it found it - so the one thing this
 ; exists to hand over is the one thing an ordinary call cannot carry. Two bytes of Stack
 ; instead of ten, as well, in a routine that runs on every block the machine ever touches.
+; ASKED FIRST, THEN WAITED ON, and that order is the whole of what makes this safe. The
+; disk raises its line when it finishes, so if it finished between the test and the WAIT
+; the line is already standing and the WAIT does nothing rather than sleeping through the
+; answer. Testing after waiting would have exactly the opposite property.
+;
+; No handler and no vector: the shell keeps the Interrupt Flag down, and a WAIT wakes on a
+; line whether or not anybody is going to answer it. It takes the line down on the way
+; past, which is what stops the next wait finding this one's line and returning at once.
 sbfsWaitDisk:
   INA 0x23
   INIB 0x01
   AND
-  BNQ sbfsWaitDisk      ; The busy bit is up, so round again.
-  RRET                  ; A is the status, with the busy bit down.
+  BRQ sbfsWaitDone      ; The busy bit is down, so there is nothing to wait for.
+  WAIT
+  BRI sbfsWaitDisk
+sbfsWaitDone:
+  RRET                  ; A is the status, with the busy bit down: AND leaves A alone.
 
 ; Copies the disk's buffer, a whole block of it, to Data Memory at DP1.
 sbfsBufferOut: