Comparing writing “Hello World” using DDT versus ASM in CP/M#
Introduction#
Using DDT, one can use the inline assembler to write small programs.
Besides DDT, CP/M also offers ASM to assemble a bit more
advanced programs. In this blogpost, I will use both programs to write a small
“Hello World” program and compare the workflow between the two procedures.
Dynamic Debugging Tool#
To write a small “Hello World” program using DDT, open up the program
and run A100 to start the inline assembler at memory position
0x100.
Type the following assembly instructions
mvi c,9
lxi d,109
call 5
ret
After writing ret, hit the return key twice to return to the command line
of DDT. Next, we run s109 to modify the memory starting at location
0x109. We enter the following values
48
65
6c
6c
6f
20
57
6f
72
6c
64
21
24
00
which corresponds to the string “Hello World!$” in ASCII-encoding, followed by a zero. Note that the ‘$’ acts as the terminating character and ‘0x00’ serves as a “padding” character. Feel free to leave it out though, it is not being used.
Note
If you would like to run this program from inside DDT, do not forget
to change ret to rst 7. Here, we use ret such that we can run
this program from the command prompt.
At this point, the program resides in memory. To exit DDT, type
g0. We return to the CP/M prompt A>. Here, we type
save 1 hello.com
which will save a single page of memory (256 bytes), starting at location
0x100 to the file HELLO.COM. We can now execute the program by
simply running hello from the command line which will show the “Hello
World!” response being written to the terminal.
ASM#
We are going to perform the same task as shown before, but now using
ASM. First, let us clean up the previous HELLO.COM file by
running
era hello.com
Next, we open the line editor ED. In its command prompt, type I
to enter insert mode and we write the following lines of assembly code.
ORG 100H
MVI C,9
LXI D,MSG
CALL 5
RET
MSG:
DB 'Hello World!$',0
After typing the 0, press CTRL+Z to write the end-of-file marker.
You will at this point return to the command line of ED. Type E
to exit and save the file to floppy disk.
Warning
It is important that you use single quotation marks to encapsulate the “Hello World!” string. Double quotation marks will not work.
From the CP/M command prompt, you can run
type hello.asm
to see the contents of the file. It should reflect the assembly code as typed in
via DDT. At this point, we are ready to start assembling the program. To
do so, run
asm hello
Warning
Do not write any extension after “hello” in the command above. In fact, writing asm hello.asm will throw an error message.
The output of the command above should yield
CP/M ASSEMBLER - VER 2.0
0117
000H USE FACTOR
END OF ASSEMBLY
which shows that the final address written to is 0x117. At this point,
three files are generated:
HELLO.PRN: A print or listing file containing a human-readable listing of the assembly source code along with the corresponding machine code and addresses. This file can be printed usingtype hello.prn.HELLO.HEX: Intel HEX file containing an ASCII-encoded hexadecimal representation of the machine code. This file can printed usingtype hello.hex.HELLO.OBJ: An object file containing the machine code generated by the assembler. This can be used in the next step to generate an executable.COMfile.
Hint
It is insightful to view the contents of HELLO.PRN and
HELLO.HEX using the TYPE command.
Finally, we are going to produce a .COM file by running
load hello
which will yield the following output
FIRST ADDRESS 0100
LAST ADDRESS 0116
BYTES READ 0017
RECORDS WRITTEN 01
At this point, the file HELLO.COM is created and one can launch it by
running hello from the CP/M prompt.
Comparison and conclusion#
For creating quick-and-dirty small assembly programs, I prefer to use
DDT as the program that is being written can be tested from the
DDT environment. The biggest downside of the inline-assembler however
is that there is no functionality for labels and one has to do a bit of mental
arithmetics to figure out all the addresses.
For larger programs, ASM in conjunction with ED is the way to
go, although ED is not a user-friendly editor. In ASM, labels
are supported by which part of job of calculating the addresses can be delegated
to the program.