--- a/doc/Low-level-commands	Sun Mar 07 04:48:37 2021 +0000
+++ b/doc/Low-level-commands	Sun Mar 07 05:19:31 2021 +0000
@@ -115,16 +115,22 @@
 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.
+this byte content to the SIM in an UPDATE RECORD command, using either absolute
+or PREVIOUS addressing mode.  The record-index argument is the same as in the
+readrec command for the absolute addressing mode, or 'prev' keyword to use the
+PREVIOUS addressing mode for writing to cyclic EFs.  The number of bytes in the
+hex data file must equal the EF record length.
+
+update-rec-imm record-index hex-string
 
-update-rec-prev hexfile
+This command works like update-rec, 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.
 
-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.
+update-rec-fill record-index fill-byte
+
+This fc-simtool command sends an UPDATE RECORD command to the SIM with payload
+equal to the specified fill byte, replicated to the record length.
 
 restore-file File_ID host-bin-file