vt52

_posts/2024-07-14-vt52.md

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+---
+layout: post
+title: "vt52, sos, ddt"
+---
+using a vt52 emulator talking to tops-10 and working in sos editor and ddt debugger, some keyboard topics quickly become apparent. the vt52 had a line feed lf key, but modern keyboards don't. so, there's not an obvious way to send lf alone. 
+
+in sos, lf prints the next line. with esc printing the previous line, lf and esc 'scroll down and up' through the file. it has a similar role in ddt, where it 'examines the next instruction'.
+
+tangentially, here's some interesting discussion from the gorin book. this belongs in the dictionary under 'unintended consequences'.
+
+_as a user of the decsystem-10 you are aware that usually you end a line by typing the carriage return key. when tops-10 sees the return key, it adds a line feed after the return. thus, usually, a program will see both a carriage return and a line feed when it reads the terminal. however, please note that tops-10 considers the line feed to be end of the line, so a program that is searching for the end of a line should not be satisfied until it finds the line feed._
+
+the tops-10 'rubout' delete key is also a topic. the modern backspace key seems to be a dead end. the delete key to the right of the enter key is correct. there's some qualifiers on that though, at least for tops-10.
+
+for line feed, the answer is to use ctrl-j. discovered this while googling 'vt52 emulator with line feed', and it instantly made sos and ddt completely usable. this was a night to day change and seems to be essential knowledge for using tops-10 now.
+
+as an additional note, ctrl-h plays an inverse role to ctrl-j, at least within ddt. there ctrl-h closes current location and moves upward, and ctrl-j does similar but moves downward. so nutshell summary -
+
+    sos
+    print upward - esc
+    print downward - ctrl-j
+
+    ddt
+    examine upward - ctrl-h
+    examine downward - ctrl-j
+
+for the 'rubout' delete key, use the one to the right of the enter key, but note tops-10 echoes the characters as they're rubbed out, surrounded by \ delimiter character. this can be disorienting at first. so for example typing 'test', then delete twice, then ' hello' results in 'test\ts\ hello' on screen. the end result is 'test hello'.
+
+the sos commands
+
+    ^ = first line
+    . = curr line
+    * = last line
+    esc = print prev, exit insert mode if inside
+    ctrl-j = print next
+    pln1:ln2 = print lines in range
+    iln1 = insert mode at ln1
+    cln1,ln2:ln3 = copy range to ln1
+    tln1,ln2:ln3 = move range to ln1
+    rln1:ln2 = delete range and insert at ln1
+    fstring$ln1:ln2 = find string in range
+    sstring$ln1:ln2 = substitute string in range
+    nx = renumber lines using step x
+    w = save
+    e = save and exit
+    es = save without line numbers
+    eq = exit without save