JMON - 8080 Version

I wanted to learn some Intel 8080 assembler programming, so I selected as a project to write a port of my JMON machine language monitor program that I wrote for the 6502 and Briel Apple 1 Replica.

After a couple of evenings of hacking I have a version running with six of the commands implemented. It is not so much a port, as an implementation from scratch of a subset of the commands and features of the 6502 version.

Some sample output is shown below (user input is in bold).

JMON Monitor 0.1 by Jeff Tranter
? ?
Valid commands:
C start end dest           Copy memory
D address                  Dump memory
F start end data...        Fill memory
G address                  Go
I                          Show info
K start end                Checksum
L                          Clear screen
R                          Examine registers
S start end data...        Search memory
T start end                Test memory
V start end dest           Verify memory
: address data...          Write to memory
= address +/- address      Hex math calculation
?                          Help

? I
JMON Monitor 0.1 by Jeff Tranter
CPU type: 8080

? R
A=00 BC=0000 DE=0000 HL=0000 F=00000110 SP=7000 PC=0000

? D 0000
0000: 32 9E 06 78 32 A0 06 79 32 A1 06 7A 32 A2 06 7B 2..x2..y2..z2..{
0010: 32 A3 06 7C 32 A4 06 7D 32 A5 06 F5 C1 79 32 9F 2..|2..}2....y2.
0020: 06 21 00 70 7C 32 A6 06 7D 32 A7 06 31 00 70 CD .!.p|2..}2..1.p.
0030: E5 02 21 A7 03 CD EC 02 3E 3F CD 92 02 CD FC 02 ..!......?......
0040: CD A1 02 CD 04 03 FE 43 C2 51 00 CD 85 02 C3 38 .......C.Q.....8
0050: 00 FE 44 C2 5C 00 CD EC 00 C3 38 00 FE 46 C2 67 ..D.\.....8..F.g
0060: 00 CD 85 02 C3 38 00 FE 47 C2 72 00 CD 4A 01 C3 .....8..G.r..J..
0070: 38 00 FE 49 C2 7D 00 CD 90 01 C3 38 00 FE 4B C2 8..I.}.....8..K.
0080: 88 00 CD 85 02 C3 38 00 FE 4C C2 93 00 CD 89 01 ......8..L......
0090: C3 38 00 FE 52 C2 9E 00 CD BC 01 C3 38 00 FE 53 .8..R.......8..S
00A0: C2 A9 00 CD 85 02 C3 38 00 FE 54 C2 B4 00 CD 85 .......8..T.....
00B0: 02 C3 38 00 FE 56 C2 BF 00 CD 85 02 C3 38 00 FE ..8..V.......8..
00C0: 3A C2 CA 00 CD 85 02 C3 38 00 FE 3D C2 D5 00 CD :.......8..=....
00D0: 85 02 C3 38 00 FE 3F C2 E0 00 CD 7B 02 C3 38 00 ...8..?....{..8.
00E0: CD AB 02 21 CA 03 CD EC 02 C3 38 00 CD 92 02 CD ...!......8.....
00F0: FC 02 CD 9C 03 D2 FC 00 CD AB 02 C9 CD AB 02 0E ................
0100: 18 E5 CD 43 03 3E 3A CD 92 02 06 10 CD FC 02 7E ...C..:........~
0110: CD 11 03 23 05 C2 0C 01 CD FC 02 E1 06 10 7E CD ...#..........~.
0120: D0 02 23 05 C2 1E 01 CD AB 02 0D C2 01 01 E5 21 ..#............!
0130: 51 06 CD EC 02 E1 CD A1 02 FE 1B C2 42 01 CD AB Q...........B...
0140: 02 C9 FE 20 CA FC 00 C3 36 01 CD 92 02 CD FC 02 ... ....6.......
0150: CD 9C 03 D2 5A 01 CD AB 02 C9 22 A8 06 CD AB 02 ....Z.....".....
0160: 2A 00 00 E5 3A A8 06 6F 3A A9 06 67 E5 3A A4 06 *...:..o:..g.:..
0170: 67 3A A5 06 6F 3A A2 06 57 3A A3 06 5F 3A A0 06 g:..o:..W:.._..
Press Space to continue, ESC to stop 

? G 0000

It builds using the AS Macro Assembler. The binary can be put on an SD card and directly loaded into the Briel Altair 8800 memory and run.

I found debugging quite straightforward by using the front panel switches of the Briel Altair 8800. In particular, the ability to step, see disassembly on the console, and set a breakpoint address, were very useful. The JMON monitor makes some things a little easier than the front panel, like dumping the contents of memory.

While my goal was to learn 8080 assembly language programming, the monitor is somewhat useful in it's current form. You can find the source code here. It is licensed under the Apache 2.0 license so you are free to use it for any purpose. As time allows, I may implement more commands and features.

Being mostly familiar with the 6502, and its successors like the 6809 and 68000, it was interesting to see how the Intel 8080 compared. It has more registers and instructions than the 6502, making some things easier, but the learning curve is perhaps higher. It also has a few unusual instructions and quirks (for example, how do you clear the carry bit?).