From Newsgroup: alt.lang.asm

I'm working through the "Introduction To DECSYSTEM-20 Assembly
Programming" book by Stephen Longo. I have the Gorin one as well but the
Longo text jumps into writing actual programs faster. There's examples
that the student is supposed to complete at the end of the
chapters. There's no given sample output and no answers so the student
is on his own. I thought I'd post some of my solutions if people want to
follow along on their PDP-10 with the MACRO-20 assembler.

@exec ch2u5
LINK: Loading
[LNKXCT CH2U5 execution]
Type in an integer: 30
Sum: 50
Dif: 10
@cont
Type in an integer: 10
Sum: 30
Dif: -10


; Chapter 2 user exercise 5
; Accept an integer from the terminal. Add the integer to
; an integer stored in the program and then subtract it
; from the same integer. Assume the integer given is 20.
title ch2u5
search monsym ; Use monitor's symbols

ac1=1 ; Label accumulators
ac2=2
ac3=3

twent: 24 ; The given number 0o24=20 dec
usrnum: 0 ; Store user's type-in number
base10: 12 ; 0o12 = 10 decimal base
ttyin: .priin ; Primary input keyboard
ttyout: .priou ; Primary output TTY

msg1: asciz /Type in an integer: / ; Prompt for user data message
sum: asciz /Sum: / ; Label sum message
dif: asciz /Dif: / ; Label difference message
errmsg: asciz /IO Error/ ; Generic error message!

start: hrroi ac1,msg1 ; Point to prompt message
psout% ; Print it to TTY

; NIN% wants device, radix to 1, 3 and puts results in ac2
move ac1,ttyin ; Point to keyboard
move ac3,base10 ; Indicate base 10
nin% ; Get user integer
erjmp error ; Handle error
movem ac2,usrnum ; Save user's num to memory

; Do the sum part of the problem now
hrroi ac1,sum ; Print sum label
psout% ; To TTY

; NOUT% wants device, number, and base in accum 1, 2, 3
move ac1,ttyout ; Indicate TTY
move ac2,usrnum ; Get user's number
add ac2,twent ; Add 20 decimal
move ac3,base10 ; Indicate base 10
nout% ; Print sum to TTY
erjmp error ; Handle error

movei ac1,15 ; Print a \r
pbout%
movei ac1,12 ; Print a \n
pbout% ; TOPS-20 wants both

; Do the difference part of the problem now
hrroi ac1,dif ; Print diff label
psout% ; To TTY

move ac1,ttyout ; Indicate TTY
move ac2,usrnum ; Retrieve user's num
sub ac2,twent ; Sub 20 decimal
move ac3,base10 ; Indicate base 10 dec
nout% ; Print diff to TTY
erjmp error ; Handle error
haltf% ; Done
jrst start ; Restart prog if desired

error: hrroi ac1,errmsg ; Indicate error
psout% ; Print to screen
haltf% ; Exit
end start ; That's it
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From Newsgroup: alt.lang.asm

Chapter 3, ch3u1 and ch3u2. I find comparisons to be confusing on this
CPU. On x86, you use "cmp", the flags are set, and you jump based on the outcome. In MACRO-20, you seem to need to test for the thing you don't
want, and then continue the execution path from there. If the condition
fires off, it "eats" next instruction, else it goes on. Very odd. See
the "check:" label below for an example of what I mean.

@exec ch3u1
LINK: Loading
[LNKXCT CH3U1 execution]
Enter an integer: 6
6
@cont
Enter an integer: -4
NEG
@cont
Enter an integer: 0
@

; Chapter 3 user exercise 1
; Accept an integer via NIN% (the text has a mistake here).
; If the int is positive, display it
; If the int is negative, print "NEG"
; Assumed to do nothing if zero.
; CAM = compare accumulator to memory location
; CAI = compare accum to immediate value
; l, g, e, n = less than, greater, equal, not equal suffixes
title ch3u1
search monsym ; Use monitor's symbols

ac1=1 ; Label accumulators
ac2=2
ac3=3

base: 12 ; 0o12 = 10 decimal
ttyin: .priin ; Primary input keyboard
ttyout: .priou ; Primary output to TTY

prompt: asciz /Enter an integer: / ; User prompt
negmsg: asciz /NEG / ; Tell user it's negative

start: hrroi ac1,prompt ; Load user prompt
psout% ; Print it to TTY

; NIN% wants device, radix to 1, 3 and puts results in ac2
; Both NIN% and NOUT% require ERJMP, which is like an "else"
move ac1,ttyin ; Get input from keyboard
move ac3,base ; Signal what base to get
nin%
erjmp done ; Don't worry about error

; NIN% will give us an integer, not the ASCII octal for it
; Compare against the integer, not the ASCII value
check: cail ac2,0 ; Is ac2's num less than 0?
jrst pos ; No/else clause, check +/0

neg: hrroi ac1,negmsg ; It's negative, load message
psout% ; and print to TTY
jumpa done ; This condition is handled

pos: caig ac2,0 ; Is ac2's num greater than 0?
jrst done ; Else, it must be 0, goto done

; nout% wants device, number, base in accum 1,2,3 and erjmp
move ac1,ttyout ; Prepare to print positive num
move ac3,base ; Load desired number base
nout% ; Output number to TTY
erjmp done ; Handle error (or not)

done: haltf% ; Either way we're done
jrst start ; User can re-run program
end start ; Tell assember we're done and
; also where the prog starts at


User exercise 2, little caculator (add/sub).

@exec ch3u2
LINK: Loading
[LNKXCT CH3U2 execution]
Enter an integer: 24
Enter another integer: 6
Enter an operation (+ or -): +
30
@cont
Enter an integer: 10
Enter another integer: 30
Enter an operation (+ or -): -
-20
@cont
Enter an integer: *
Error: invalid input or operation
@

; Chapter 3 user exercise 2
; Get two integers via NIN%, PBIN% an operation as earlier
; in the text (only use + or -). Perform the operation and
; output the answer. Give error message if operation is
; not + or -.
title ch3u2
search monsym ; Use monitor's symbols

ac1=1 ; Label accumulators
ac2=2
ac3=3
op=7 ; Save user's operation

int1: 0 ; User's first num
int2: 0 ; User's second num

base: 12 ; 0o12 = 10 decimal
ttyin: .priin ; Keyboard
ttyout: .priou ; Primary output to TTY
add: "+" ; Mathmatical operations
sub: "-"

; User prompts and error messages
getnum: asciz /Enter an integer: /
getnm2: asciz /Enter another integer: /
errmsg: asciz /Error: invalid input or operation/
opmsg: asciz /Enter an operation (+ or -): /

start: hrroi ac1,getnum ; Load prompt for int1
psout% ; And print it

; NIN% wants device, radix to 1, 3 and puts results in ac2
; Both NIN% and NOUT% require ERJMP, which is like an "else"
move ac1,ttyin ; Prepare to get from kbd
move ac3,base ; Requested number base
nin% ; Fetch number to ac2
erjmp error ; Handle error
movem ac2,int1 ; Save user's 1st num

hrroi ac1,getnm2 ; Load prompt for int2
psout% ; And print it to TTY

; NIN% wants device, radix to 1, 3 and puts results in ac2
move ac1,ttyin ; Prepare to get from kbd
move ac3,base ; Requested number base
nin% ; Fetch number to ac2
erjmp error ; If error
movem ac2,int2 ; Save second number

; Numbers 1 and 2 are fetched. Now get the operation
hrroi ac1,opmsg ; Prompt for operation
psout% ; Print to TTY
pbin% ; Get char, either + or -
move op,ac1 ; Save it
pbin% ; Eat \r else stray input
pbin% ; Eat \n too

; Check the operation is a valid one (+ or - only)
camn op,add ; Is it '+'?
jrst plus ; It is equal, do plus
camn op,sub ; Is it '-'?
jrst minus ; It is equal, do minus
jumpa error ; Don't care if otherwise

plus: move ac2,int1 ; Fetch int1
add ac2,int2 ; Add int2 to it
jumpa disp ; Ready to display

minus: move ac2,int1 ; Fetch int1
sub ac2,int2 ; Subtract it from int1

; Display output section. Print the result that we calculated
; above; it is in accumulator 2. We're here by jump or fall-thru.
disp: move ac1,ttyout ; Output device is TTY
move ac3,base ; Load base
; nout% wants device, number, base in accum 1,2,3 and erjmp
nout% ; Print result, in ac2
erjmp error ; Handle error
haltf% ; Else we're finished here
jrst start ; Re-run/cont the program

error: hrroi ac1,errmsg ; Load error message
psout% ; And print to TTY
haltf% ; Exit to monitor
end start ; That's it, indicate start
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From Newsgroup: alt.lang.asm

jayjwa,

In MACRO-20, you seem to need to test for the thing you don't
want, .... If the condition fires off, it "eats" next instruction,
else it goes on. Very odd.

Its not as odd as you might think : imagine the possibly eaten instruction
is a jump. In that case you jump on "the thing you want".

Yes, its a bit of a mind-trick to think about it that way. :-)

You might also think of it as "skip next instruction if true". iow, if the "eaten" instruction is again a jump, the program will take that jump if the comparision is not true.

Ofcourse, nothing stops you from replacing that jump with the (re)setting of
a flag in a register. :-)

Regards,
Rudy Wieser


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From Newsgroup: alt.lang.asm

ch3u3
Lots of comparisons in this one. It still seems backwards.
@exec ch3u3
LINK: Loading
[LNKXCT CH3U3 execution]
Enter a character (A, B to echo, C to quit): A
A
Enter a character (A, B to echo, C to quit): B
B
Enter a character (A, B to echo, C to quit): D
Enter a character (A, B to echo, C to quit): F
Enter a character (A, B to echo, C to quit): C
Program complete
@cont
Enter a character (A, B to echo, C to quit): *
Enter a character (A, B to echo, C to quit): C
Program complete
@

Last night I found that DDT can display with your symbols already
inserted, like this:
@debug ch3u3
LINK: Loading
[LNKDEB DDT execution]
DDT
ch3u3$:
start$b $g

START/ MOVEI ITER,5

gdb seems so friendly and luxurious after using DDT. The text doesn't
warn about dangling newlines (actually \r\n in TOPS-10/20) but your
program will run incorrectly under EXEC but fine under DDT if you don't
handle them. The text for TOPS-10 mentions them and of course C
programmers have been bit by them at some point.

comment $
Chapter 3 user exercise 3
PBIN% a character. PBOUT% the character only if it is A or B.
Place the above in a loop that terminates after the 5th PBIN%
or if the letter C is entered.
$
title ch3u3
search monsym ; Use monitor's symbols

ac1=1 ; Label accumulators
iter=7 ; Iterator for loopage

char: "X" ; User's character

; User prompts and messages
getchr: asciz /Enter a character (A, B to echo, C to quit): /
compl: asciz /Program complete/

start: movei iter,5 ; Init iterator for loops
loop: hrroi ac1,getchr ; Load prompt
psout% ; And print to TTY

pbin% ; Get char to ac1
movem ac1,char ; Save user's char

; Input will be char\r\n, deal with \r\n on input stream. Oddly,
; the text does not tell you this and your program will run fine
; under DDT while debugging but will fail under EXEC.
pbin%
pbin%

; Now look at what character we have
move ac1,char
cain ac1,101 ; Compare to A (0o101)
jrst disp ; It's A
cain ac1,102 ; Compare to B
jrst disp ; It's B
cain ac1,103 ; Compare to C
jrst done ; C - exit loop to done
jumpa pass ; None of these, no output

disp: pbout% ; Display char in ac1
movei ac1,15 ; Load \r
pbout%
movei ac1,12 ; Load \n
pbout% ; End in \r\n

; Check loop condition. Do we restart the process?
pass: subi iter,1 ; Cut 1 from our looper
jumpn iter,loop ; Not zero? repeat loop

done: hrroi ac1,compl ; Signal program complete
psout%
haltf% ; Exit to monitor
jrst start ; Restartable program
end start ; That's it, halt assembler
--
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"The Internet should always be the Wild West!"
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