--- a/doc/Low-level-commands	Wed Feb 17 19:35:56 2021 +0000
+++ b/doc/Low-level-commands	Wed Feb 17 19:54:57 2021 +0000
@@ -96,9 +96,34 @@
 Writing commands
 ================
 
-update-bin
-update-bin-imm
-update-rec
+update-bin offset hexfile
+
+This fc-simtool command reads a hex data file (an ASCII text file containing
+only hex byte values and nothing else, with or without white space between
+bytes, newlines treated as any other white space) and sends this byte content
+to the SIM in an UPDATE BINARY command.  The offset argument is the same as in
+the readbin command.  The length is the number of bytes read from the hex data
+file.
+
+update-bin-imm offset hex-string
+
+This command works like update-bin, but the bytes to be written are given as a
+hex string direct argument (like an immediate operand in assembly languages),
+rather than via a hex data file.
+
+update-rec record-index hexfile
+
+This fc-simtool command reads a hex data file (just like update-bin) and sends
+this byte content to the SIM in an UPDATE RECORD command, using absolute
+addressing mode.  The record-index argument is the same as in the readrec
+command.  The number of bytes in the hex data file must equal the EF record
+length.
+
+update-rec-prev hexfile
+
+This fc-simtool command is like update-rec, but the UPDATE RECORD command sent
+to the SIM uses the PREVIOUS addressing mode, and there is no record number.
+This form is needed in order to write to cyclic EFs.
 
 restore-file