s6.s: add comments

src/sys/s6.s

@@ -99,6 +99,8 @@ namei: 0
    jmp 1b		"  yes: keep going
    jmp namei i		" Didn't find it, return zero in AC (without skip)
 t = t+2
+
+		" NOTE: iget/iput share "temp" vars!!!
                         " Given an i-number in AC, fetch that i-node
                         " from disk and store it in the inode buffer
 iget: 0
@@ -113,49 +115,50 @@ iget: 0
    cll; mul; 12		" Multiply the i-num within the block by 12
    lacq			" to get the offset into the block
    tad dskbufp		" Add on the base of the disk buffer
-   dac 9f+t
+   dac 9f+t		" save pointer into dskbuf in "t0"
    dac .+2
    jms copy; ..; inode; 12	" Copy 12 words from buffer to inode
    jmp iget i		" and return
 
 iput: 0
-   lac 9f+t+1
+   lac 9f+t+1		" get block number saved by iget
-   jms dskrd
+   jms dskrd		" read the block
-   law inode-1
+   law inode-1		" pointer to "in core" inode
-   dac 8
+   dac 8		" in index 8
    -1
-   tad 9f+t
+   tad 9f+t		" get saved pointer into buffer
-   dac 9
+   dac 9		" in index 9
-   -12
+   -12			" 12 word loop count
-   dac 9f+t+2
+   dac 9f+t+2		" in t2
 1:
-   lac 8 i
+   lac 8 i		" fetch source word
 
 "** 01-s1.pdf page 36
 
-   sad 9 i
+   sad 9 i		" differs from disk?
-   skp
+   skp			"  no:
-   jmp 2f
+   jmp 2f		"   yes: 
-   isz 9f+t+2
+   isz 9f+t+2		" no difference: done?
-   jmp 1b
+   jmp 1b		"  no: keep going
-   jmp iput i
+   jmp iput i		" yes: return w/o writing to disk
-2:
+2:			" here with in-core inode changed
-   -1
+   -1			" back up both pointers
    tad 8
    dac 8
    -1
    tad 9
    dac 9
 1:
-   lac 8 i
+   lac 8 i		" fetch word
-   dac 9 i
+   dac 9 i		" copy to dskbuf
-   isz 9f+t+2
+   isz 9f+t+2		" done?
-   jmp 1b
+   jmp 1b		"  no: keep going
-   lac 9f+t+1
+   lac 9f+t+1		" get block number
-   jms dskwr
+   jms dskwr		" write back to disk
-   jmp iput i
+   jmp iput i		" return
 t = t+3
 
+	" NOTE! dget/dput share "temp" vars
 	" allocate directory entry
 	" AC/ entry number
 dget: 0
@@ -164,30 +167,31 @@ dget: 0
    dac 9f+t			" save in t0
    jms pget			" get free disk block
    dac 9f+t+1			" save in t1
-   jms dskrd
+   jms dskrd			" read block
-   lac 9f+t
+   lac 9f+t			" get word number
-   and o77
+   and o77			" get word within block
-   tad dskbufp
+   tad dskbufp			" make pointer into dskbuf
-   dac 9f+t+2
+   dac 9f+t+2			" save in t2
-   dac .+2
+   dac .+2			" save as copy source
-   jms copy; ..; dnode; 8
+   jms copy; ..; dnode; 8	" copy entry into dnode
-   lac 9f+t
+   lac 9f+t			" get word number
-   tad d8
+   tad d8			" of end of new node
-   jms betwen; d0; i.size
+   jms betwen; d0; i.size	" already allocated?
-      skp
+      skp			"  no
-   jmp dget i
+   jmp dget i			"   yes: return
-   jms dacisize
+   jms dacisize			" store new file size
-   dzm d.i
+   dzm d.i			" mark new entry as free
-   jmp dget i
+   jmp dget i			" return
 
+	" write directory entry (back) to disk
 dput: 0
-   lac 9f+t+1
+   lac 9f+t+1			" get disk block number back
-   jms dskrd
+   jms dskrd			" read into dskbuf
-   lac 9f+t+2
+   lac 9f+t+2			" get pointer into dskbuf
-   dac .+3
+   dac .+3			" save as copy dest
-   jms copy; dnode; ..; 8
+   jms copy; dnode; ..; 8	" copy into dskbuf
-   lac 9f+t+1
+   lac 9f+t+1			" get disk block number
-   jms dskwr
+   jms dskwr			" write back to disk
    jmp dput i
 
 t = t+3
@@ -262,6 +266,10 @@ pget: 0
    jmp pget i				" return it
 t = t+3
 
+	" write to file referenced by loaded inode
+	" AC/ file offset
+	"    jms iwrite; addr; count
+
 iwrite: 0
    dac 9f+t				" save arg in t0
    lac iwrite				" load return address
@@ -273,14 +281,14 @@ iwrite: 0
    dac iwrite				" save as iwrite instruction
    jmp 1f
 
-	" iread from file referenced by loaded inode
+	" read from file referenced by loaded inode
 	" AC/ file offset
 	"    jms iread; addr; count
 iread: 0
    dac 9f+t				" save offset in t0
    lac cnop				" get nop
    dac iwrite				" save as iwrite instruction
-1:
+1:					" common code for iread/iwrite
    -1
    tad iread i				" get word before return addr
    dac 10				" store in index 10 & 11
@@ -289,20 +297,14 @@ iread: 0
    lac iread i				" load addr
    dac 9f+t+1				" save in t1
    isz iread				" increment return addr
-   lac o70000
+   lac o70000				" get max file size
    xct iwrite				" skip if write
    lac i.size				"  read: get file size
    cma
-   tad 9f+t				" add offset
+   tad 9f+t				" subtract offset
-   cma
+   sna					" offset == size?
-   jms betwen; d0; 9f+t+1
+   jmp iread i				"  yes, return
-      lac 9f+t+1
+   dac 9f+t+1				" save size-offset in t1
-   dac 9f+t+2
-   cma
-   tad d1
-   sna
-   jmp iread i
-   dac 9f+t+1
 1:
    lac 9f+t
    jms pget
@@ -312,7 +314,7 @@ iread: 0
    and o77
    tad dskbufp
    tad dm1
-   xct iwrite
+   xct iwrite				" skip if write
    jmp .+3
    dac 10
 cskp: