Improve sys annotations

Finally understand location 7 (incremented at 60Hz)
	setting it to -1 causes clock interrupt every jiffy
Improve comments in dsktrans
Clarify cnop/cskp usage in iread/iwrite code
	iwrite and iread have common code
	entry at iwrite save return address at iread
		and overwrites return address to "skp" inst
	entry at iread puts "nop" inst at iwrite
	"xct iwrite" skips on write entry

src/sys/s6.s

@@ -270,12 +270,10 @@ t = t+3
 	" AC/ file offset
 	"    jms iwrite; addr; count
 
-iwrite: 0
+iwrite: 0				" set to skp (or nop by iread)
    dac 9f+t				" save arg in t0
    lac iwrite				" load return address
-
 "** 01-s1.pdf page 38
-
    dac iread				" save as iread return addr
    lac cskp				" load skip instruction
    dac iwrite				" save as iwrite instruction
@@ -287,7 +285,7 @@ iwrite: 0
 iread: 0
    dac 9f+t				" save offset in t0
    lac cnop				" get nop
-   dac iwrite				" save as iwrite instruction
+   dac iwrite				" save in iwrite return location
 1:					" common code for iread/iwrite
    -1
    tad iread i				" get word before return addr
@@ -322,17 +320,17 @@ iread: 0
    tad dm1
    xct iwrite				" skip if write
    jmp .+3
-   dac 10
-cskp:
+   dac 10				" here on read
+cskp:					" literal for store to iwrite
    skp
-   dac 11
+   dac 11				" here on write
 2:
    lac 11 i
    dac 10 i
    isz 9f+t
    isz 9f+t+1
    jmp 3f
-      xct iwrite
+      xct iwrite			" skip if write
       jmp 4f
       lac 9f+t
       jms betwen; d0; i.size
@@ -348,7 +346,7 @@ cskp:
    and o77
    sza
    jmp 2b
-   xct iwrite
+   xct iwrite			" skip if write
    jmp 1b
    lac 9f+t+3
    jms dskwr