I got a suitably dog-eared copy of 8080/Z80 Assembly Language Techniques for Improved Programming that covers the development of a system monitor in chapter 6.

The code is also available here, but the book breaks it down into stages so you can build up and debug the functionality step by step. This is invaluable since my 8080 byte-code interpreter is riddled with bugs!

There was some additional work needed before even getting through the first exercise in attaching the console. I needed a way to interface the virtual UART to the 8080 and the most elegant way of doing this was via the input/output ports. The first 8 were assigned to the expansion board, but the rest have now been assigned as follows:

The system's zero page is not addressable by the 8080, so 52 ports are mapped to this memory space via the ports. The first 44 bytes have read/write access and the last 8 are read only.

PORT  $NAME          ADDR   DESCRIPTION
----  -----          ----   -----------
#SOUND (read/write)
12    $PCMPGH        0x8C # PCM page high water - stop page
13    $PCMPGL        0x8D # PCM page low water - current page
14    $WAVE0         0x8E
15    $ATTACK0       0x8F
16    $DECAY0        0x90
17    $SUSTAIN0      0x91
18    $RELEASE0      0x92
19    $NOTE1L        0x93 # note interval low (FREQL)
20    $NOTE1H        0x94 # note interval high (FREQH)
21    $WAVE1         0x95 # wave table entry (SQRWAV/SAWWAV: 0000WWWW)
22    $ATTACK1       0x96 # ADSR attack (positive 4-bit packed to 8-bits *8: 0AAAA000)
23    $DECAY1        0x97 # ADSR decay (negative 4-bit packed to 8-bits *8: 1DDDD000)
24    $SUSTAIN1      0x98 # ADSR sustain (negative 4-bit packed to 8-bits: 1111SSSS)
25    $RELEASE1      0x99 # ADSR release (negative 4-bit packed to 8-bits *8: 1RRRR000)
26    $NOTE2L        0x9A
27    $NOTE2H        0x9B
28    $WAVE2         0x9C
29    $ATTACK2       0x9D
30    $DECAY2        0x9E
31    $SUSTAIN2      0x9F
32    $RELEASE2      0xA0
33    $NOTE3L        0xA1
34    $NOTE3H        0xA2
35    $WAVE3         0xA3
36    $ATTACK3       0xA4
37    $DECAY3        0xA5
38    $SUSTAIN3      0xA6
39    $RELEASE3      0xA7
#CONSOLE (read/write)
40    $CONL          0xA8 # console left border
41    $CONR          0xA9 # console right border
42    $CONX          0xAA # console column
43    $CONY          0xAB # console row
44    $CONH          0xAC # console height from bottom
45    $CONB          0xAD # console backspace stop
46    $CONC          0xAE # console cursor char
47    $CONF          0xAF # console font
48    $VSTART        0xB0 # start of video display
48    $MODE          0xB1 # video mode to set
50    #SPARE         0xB2 (1)
51    $KTO           0xB3 # max kbd idle count
52    $KSRDIDX       0xB4
53    $KSWRIDX       0xB5
54    $KCRDIDX       0xB6
55    $KCWRIDX       0xB7
#PROTECTED STATE (read-only)
56    $KBSTAT        0xB8 # kbd status
57    #SPARE         0xB9 (1)
58    $BLOCK         0xBA # block count, 0 to 175/160/128
59    $FRAME         0xBB # frame count, -5,-4 to 0
60    $TIME0         0xBC # 15tps, max 90 - 6s
61    $TIME1         0xBD # 10tpm, max 120 - 12m
62    $TIME2         0xBE # 5tph, max 120 - 1d
63    $TIME3         0xBF # 1tpd, max 256 - 0.7y

 The console provides a decoded keyboard input and a simple text terminal output to make interfacing easy for the system monitor.

The second exercise in the monitor development was the memory dump command. This is now working after debugging the associated 8080 instructions and arithmetic functions. The following animated GIF demonstrates dumping memory locations 0-300 in real time.