diff --git a/src/sys/s1.s b/src/sys/s1.s
index 80bc1c0..8dcf364 100644
--- a/src/sys/s1.s
+++ b/src/sys/s1.s
@@ -21,61 +21,61 @@ orig:
    jmp 1f+1
    1f
 1: 0
-   iof
-   dac u.ac
+   iof				" interrupts off
+   dac u.ac			" save user AC
    lacq
-   dac u.mq
+   dac u.mq			" save user MQ
    lac 8
-   dac u.rq
+   dac u.rq			" save user auto-index location 8
    lac 9
-   dac u.rq+1
-   jms copy; 10; u.rq+2; 6
-   lac 1b
-   dac u.rq+8
+   dac u.rq+1			" save user auto-index location 9
+   jms copy; 10; u.rq+2; 6	" save user auto-index locations 10-15
+   lac 1b			" load user PC after system call
+   dac u.rq+8			" save user PC
+   -1				" load -1
+   dac .savblk			" set "save" flag (cleared by disk I/O?)
+   dac .insys			" set "in system" flag
+   lac uquant			" load user quantum count
+   jms betwen; d0; maxquant	" check if between 0 & maxquant??
+      jms swap			" no: swap processes
+   ion				" interrupts on
    -1
-   dac .savblk
-   dac .insys
-   lac uquant
-   jms betwen; d0; maxquant
-      jms swap
-   ion
-   -1
-   tad u.rq+8
-   jms laci
-   jms betwen; o20001; swn
-      jmp badcal
-   tad swp
-   dac .+1
-   jmp .. i
+   tad u.rq+8			" get address of system call
+   jms laci			" load AC indirect??
+   jms betwen; o20001; swn	" range check
+      jmp badcal		" bad system call
+   tad swp			" add system call table base
+   dac .+1			" save as next instruction
+   jmp .. i			" dispatch system call
 
 . = orig+0100
    jmp coldentry
    jms halt
 
 okexit:
-   dzm: u.ac
-sysexit:
-   ion
-   lac .savblk
-   sza
-   jmp 1f
-   jms copy; sysdata; dskbuf; 64
+   dzm u.ac			" 'OK' system call exit: clear user AC
+sysexit:			" common system call exit code
+   ion				" enable interrupts
+   lac .savblk			" load "save" flag
+   sza				" is zero (cleared by disk I/O)?
+   jmp 1f			" no: no disk I/O done?
+   jms copy; sysdata; dskbuf; 64 " copy system data to disk buffer
    cla
-   jms dskio; 07000
+   jms dskio; 07000		" save to disk?
 1:
-   dzm .insys
+   dzm .insys			" clear "in system call" flag
    jms chkint
       skp
-   jmp .save
-   jms copy; u.rq+2; 10; 6
-   lac u.rq+1
+   jmp .save			" dump core??
+   jms copy; u.rq+2; 10; 6	" restore auto-index locations 10-15
+   lac u.rq+1			" restore auto-index location 9
    dac 9
-   lac u.rq
+   lac u.rq			" restore auto-index location 8
    dac 8
-   lac u.rq
+   lac u.mq			" restore MQ register
    lmq
-   lac u.ac
-   jmp u.rq+8 i
+   lac u.ac			" restore AC register
+   jmp u.rq+8 i			" return to user
 
 swap: 0
    ion
@@ -129,21 +129,21 @@ swap: 0
    jmp swap i
 t = t+1
 
-swp:
-   jmp .
+swp:			" system call dispatch table
+   jmp .		" base instruction
    .save; .getuid; .open; .read; .write; .creat; .seek; .tell
    .close; .link; .unlink; .setuid; .rename; .exit; .time; .intrp
    .chdir; .chmod; .chown; badcal; .sysloc; badcal; .capt; .rele
    .status; badcal; .smes; .rmes; .fork
 swn:
-   .-swp-1 i
+   .-swp-1 i		" count of system calls, plus indirect!
 
 .intrp:
    lac u.ac
    dac u.intflg
    jmp okexit
 
-.sysloc:
+.sysloc:		" "sysloc": syscall to return system addresses
    lac u.ac
    and o17777
    jms betwen; d1; locn
@@ -154,7 +154,7 @@ swn:
    dac u.ac
    jmp sysexit
 
-locsw:
+locsw:			" table of system data structures for "sysloc" call
    lac .
    iget; inode; userdata; sysdata; copy; copyz; betwen; dskrd
    dskwr; dskbuf; dpdata; namei; pbsflgs; alloc; free; dspdata
diff --git a/src/sys/s2.s b/src/sys/s2.s
index 9d7af58..3424c44 100644
--- a/src/sys/s2.s
+++ b/src/sys/s2.s
@@ -53,9 +53,9 @@
    jms iput
    jmp okexit
 
-.getuid:
+.getuid:			" getuid system call
    lac u.uid
-   dac u.ac
+   dac u.ac			" return u.uid in user AC
    jmp sysexit
 
 .seek:
@@ -146,12 +146,12 @@
    jms iput
    jmp sysexit
 
-.setuid:
-   lac u.uid
-   sma
-   jms error
-   lac u.ac
-   dac u.uid
+.setuid:			" setuid system call
+   lac u.uid			" load current user id
+   sma				" negative?
+   jms error			" no: error!!
+   lac u.ac			" load user AC
+   dac u.uid			" save as new uid
    jmp sysexit
 
 .rename:
@@ -168,11 +168,14 @@
    jms copy; 1:0; d.name; 4
    jmp okexit
 
+	" time system call returns line (mains) frequency ticks since boot?
+	" note: returns uptime!?
+	" at 60Hz, 36 bits would last 36+ years!
 .time:
-   lac s.tim
-   dac u.ac
-   lac s.tim+1
-   dac u.mq
+   lac s.tim			" load high order bits
+   dac u.ac			" return in AC
+   lac s.tim+1			" load low order bits
+   dac u.mq			" return in MQ
    jmp sysexit
 
 .chdir:
diff --git a/src/sys/s7.s b/src/sys/s7.s
index c5b307b..46e53ac 100644
--- a/src/sys/s7.s
+++ b/src/sys/s7.s
@@ -1,7 +1,7 @@
 "** 01-s1.pdf page 41
 " s7
 
-pibreak:
+pibreak:			" priority interrupt processing "chain"
    dac .ac	"** CROSSED OUT....
 
    dpsf
@@ -25,15 +25,15 @@ pibreak:
    dac dpwrite
    jmp piret	"** END OF CROSSOUT
 
-1: clsf
-   jmp 1f
+1: clsf			" clock overflow (line frequency ticks)?
+   jmp 1f		"  no
 
-   lpb
-   dac pbsflgs
-   isz s.tim+1
-   skp
-   isz s.tim
-   isz uquant
+   lpb			" load display push buttons
+   dac pbsflgs		" save
+   isz s.tim+1		" increment low order tick count
+   skp			"  no overflow, skip second increment
+   isz s.tim		"   low order overflowed, increment high order count
+   isz uquant		"    increment user quantum counter
 	"** written: ttydelay -> ttyd1
 	"** written: ttyrestart -> ttyres1
 cnop: