Hand-typed version of the cat source code with my own comments to

explain what it is doing.

src/cmd/cat.s

@@ -0,0 +1,146 @@
+" cat
+
+   lac 017777 i			" exit if four argument words only?
+   sad d4
+   jmp nofiles
+   lac 017777
+   tad d1			" why not tad d5 (add 5)?
+   tad d4
+   dac name			" name = *(017777) + 5
+
+loop:
+   sys open; name; 0; 0		" open file, get fd back
+   spa				" exit if fd == -1
+   jmp badfile
+   dac fi			" save file descriptor in fi
+
+1:
+   jms getc			" get a character in AC
+   sad o4
+   jmp 1f
+   jms putc			" write the character on stdout
+   jmp 1b
+
+1:
+   lac fi
+   sys close
+
+loop1:
+   -4
+   tad 017777 i
+   dac 017777 i
+   sad d4
+   jmp done
+   lac name
+   tad d4
+   dac name
+   jmp loop
+
+badfile:
+   lac name
+   dac 1f
+   lac d8
+   sys write; 1:0; 4
+   lac d8
+   sys write; 1f; 2
+   jmp loop1
+
+1: 040;077012
+nofiles:
+   lac d8
+   sys write; 1f; 5		" write 5 words to fd 8?
+   sys exit			" and exit
+
+1: <no>; 040;  <fi>;<le>;<s 012
+
+done:
+   lac noc
+   sns
+   sys exit
+   and d1
+   sna cla
+   jmp 1f
+   jms putc
+   jmp done
+1:
+   lac noc
+   rcr
+   dac 1f
+   lac fo			" load fd 1, stdout
+   sys write; iopt+1; 1:..
+   sys exit
+
+getc: 0
+   lac ipt
+   sad eipt
+   jmp 1f
+   dac 1f
+   add o400000
+   dac ipt
+   ral
+   lac 2f i
+   szl
+   lrss 9
+   and o177			" keep the lowest 7 bits
+   sna
+   jmp getc+1
+   jmp getc i			" return from subroutine
+1:
+   lac fi
+   sys read; iipt+1; 64
+   sna
+   jmp 1f
+   tad iipt
+   dac eipt
+   lac iipt
+   dac ipt
+   jmp getc+1
+1:
+   lac 64
+   jmp getc i			" return from subroutine
+
+putc: 0
+   and o177			" keep the lowest 7 bits
+   dac 2f+1
+   lac opt
+   dac 2f
+   add o400000
+   dac opt
+   spa
+   jmp 1f
+   lac 2f i
+   xor 2f+1
+   jmp 3f
+1:
+   lac 2f+1
+   alss 9
+3:
+   dac 2f i
+   isz noc
+   lac noc
+   sad d128
+   skp
+   jmp putc i
+   lac fo			" load fd 1, stdout
+   sys write; iopt+1; 64
+   lac iopt
+   dac opt
+   dzm noc
+   jmp putc i
+2: 0;0
+ipt: 0
+eipt: 0
+iipt: .+1; .=.+64		" 64 word input buffer
+fi: 0
+opt: .+2
+iopt: .+1; .=.+64		" 64 word output buffer
+noc: 0
+fo: 1				" output file descriptor, fd 1 is stdout
+name: 0				" not in the original
+
+d1: 1				" octal and decimal constants
+o4:d4: 4
+d8: 8
+o400000: 0400000
+o177: 0177
+d128: 128