Sorbus System Monitor

This is intended to be the "go-to" system monitor to use. There are two different versions to invoke:

NMOS 6502 version in Bootsector 2, as part of "NMOS 6502 toolkit". This version omits the usage of the ROR opcode deliberately, so it can be used even with a 6502 Rev.A chip. There is an extra chapter that discusses the different 6502 variants (including the Rev.A) a bit more in detail
the standard version included in ROM and invoked either by pressing "m" in the reset menu or by invoking BRK #$0e (or BRK #$00, if user vector UVBRK wasn't changed)

Invoking

When the System Monitor is started it looks like this:

Sorbus System Monitor via menu
   PC  BK AC XR YR SP NV-BDIZC
 ~FFF2 01 00 00 00 FF 00100110
>_  <-- cursor

Note how it tells you that it was launched via the menu. This can be also an IRQ, NMI or BRK, if the vectors for those were not changed from the system default. The line starting with "PC" is just information on what is shown below:

Entry	Meaning
PC	Program Counter
BK	BanK (not a CPU register but a system register) (not available on NMOS version)
AC	ACcumulator
XR	X index Register
YR	Y index Register
SP	Stack Pointer
NV-BDIZC	processor flags: Negative, oVerflow, Brk, Decimal, Interrupt disable, Zero, Carry

Those registers are not the actual CPU registers, but shadow registers to which the data was copied upon startup. When leaving the monitor with the go (G) command, the data will be written back to those registers. The tilde (~) just denotes that the following line contains registers. The greater-than-sign (>) is the entry prompt.

Line Input

The line input is capable of minimal command line editing

Key	Function
Home	go to start of input
Ctrl+A	go to start of input
End	go to end of input
Ctrl+E	go to end of input
Backspace	delete character left of cursor
Delete	delete character under the cursor
Ctrl+D	delete character under the cursor
Ctrl+K	delete everything right from the cursor
Ctrl+C	cancel input

(This is a kernel function that also used by OSI BASIC for example.)

Supported Commands

Command	Function	Comment
R	dump shadow registers
G	go
~	edit shadow registers
M	dump memory
:	edit memory
D	disassemble
A	assemble
C	compare
F	fill
H	hunt (find)
T	transfer (copy)
;	handle papertape input	(not available on CMOS version)
BR	read sector from internal drive
BW	write sector to internal drive
L	load file from CP/M filesystem	(not available on NMOS version)
S	save file to CP/M filesystem	(not available on NMOS version)
$	show directory of CP/M filesystem	(not available on NMOS version)

(Input is case insensitive.)

Command Examples

The available commands are better explained by examples then some abstract syntax notation. Also is the behaviour very close related to monitors related to Commodore, like the monitor in the C264 series, the C128 or those which are known from cartridges like Action Replay or Final Cartridge III.

Addresses and data are always entered in hex. Addresses need to specified with 1 to 4 digits, data with 1 or 2 digits. When addresses are specified with 4 digits and data with 2 digits, spaces are optional. In the examples those are included for better readability.

Note that an end address is specified "the Commodore way" as "plus 1", so the parameters "0400 0480" will only span the area starting at $0400 and using $047f as the last byte.

So these commands are equivalent:

>m 0400 0480
>m04000480
>m 400 480

Hexdump memory

>M 0400 0480
>m 0400 0480
>m04000480
>m 400 480

Those command all are equivalent and show the memory from 0400 to 047f:

 :0400  A2 00 A0 00 B9 31 04 9D  00 02 C8 C0 0B 90 02 A0  .....1..........
 :0410  00 E8 D0 F0 A9 E2 20 03  FF A9 94 20 03 FF AE 02  ...... .... ....
 :0420  DF BD 00 02 20 03 FF 20  00 FF C9 03 D0 E6 6C FC  .... .. ......l.
 :0430  FF 80 82 8C 90 94 98 9C  A4 AC B4 BC 00 00 00 00  ................
 :0440  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ................
 :0450  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ................
 :0460  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ................
 :0470  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ................

So spaces are optional as long as you use 4 digits for addresses and 2 digits for data. After running this you can scroll up/down through memory by pressing up/down on the keyboard:

>:0470 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

ASCII output is missing here. Memory can be edited here.

Disassemble

>d e000
>d e000 e100

The first one disassembles 20 instructions. You can enter d without an address again to continue disassembling for another 20 instructions. The second one will disassemble the memory area from $e000 to $e100.

Assemble

>a 0400 jmp (fffc)

It will turn to this for a successful assembly:

 > 0400   6C FC FF    JMP   ($FFFC)
>A 0403               _  <-- cursor

See how the assembler has already prepared the next line for entering.

Compare

>c 1000 2000 4000

Compares the area from $1000 to $1fff with the area from $4000 to $4fff. All addresses that do not match will be printed out, where the address of the first area will be used.

Fill

>f 1000 2000 bd

Fills the memory from $1000 to $1fff with the value of $bd.

Hunt

>h 0400 1000 f2 ff

Searches for the byte combination $ff, $f2 in the area from $0400 to $0fff. When such a combination is found, the address of the first byte is printed.

Transfer

>t 1000 2000 c000

Transfer (or rather copy) the memory area from $1000 to $1fff to the area of $c000 to $cfff.

Note that data wil be copied from start to end address, so

>t 1000 2000 1001

will write the byte at $1000 to every location until $2000.

Block Read/Write

>br 0040 0400
>br 40 400

Will read a 128 bytes sector from the internal drive to memory. In this case the first sector of the second bootblock containing the NMOS 6502 toolkit. This internal drive has a capacity of 4MB in 32768 sectors from $0000 to $7fff.

Technically we're using sectors of 128 bytes here, not blocks combining 16 sectors (2048 bytes) to a single block. But the S is already assigned to "Save". (See below.) Also, the commands BR and BW were used by the monitor of the Action Replay Cartridge as well.

CP/M Filesystem Load/Save

>la asmprint.sx4 0400
>l a asmprint.sx4 0400
>s2 memory.bin 1000 2000
>s 2 memory.bin 1000 2000

Loads the file "asmprint.sx4" to memory location $0400 from the user-id $A (or 10 decimal). For save you also need to add an end address. This is shown in the third and forth example where user id $2 is used. Again, the addresses are specified in "Commodore format" excluding the last byte, so "1000 2000" saves the memory from $1000 to $1fff.

Note that the filesystem implementation always works on full sectors, so even a save like "1000 1001" will only save a single byte (also noted as this in the directory), it will actually save (and load) the area from $1000 to $107f.

Also note, while spaces are typically optional in the monitor, to separate the filename from the rest of the input, they are essential.

Since kernel routines are used, load and save will always use RAM. Supported areas are $0400-$cfff and $df80-$ffff. $0004-$03ff will do unpredictable stuff, as this area is swapable. $d000-$df7f will just cause an error.

Differences between 65SC02 in ROM and NMOS 6502 toolkit versions

Topic	JAM ROM	NMOS 6502 toolkit
opcodes	CMOS 65SC02 opcodes	NMOS 6502 opcodes
BRK	CPU correct 2-Byte BRK #$xx	adjusted to historical 1-Byte BRK
storage	save to/load from CP/M-fs	not supported
IRQ	handled via kernel to check for BRK	passed almost directly to monitor
banking	bank shadow register used	bank shadow register ignored

If the bank register is set to $00, then using "G" command will copy the BIOS from ROM to RAM in process. Without it the code for executing would not work.

Also upon startup of the NMOS 6502 toolkit, you will be greeted with the CPU detected upon cold boot. It should be one of the following:

6502 CPU features: NMOS
65SC02 CPU features: CMOS
65C02 CPU features: CMOS BBSR
65CE02 CPU features: CMOS BBSR Z-Reg
65816 CPU features: CMOS 16bit
6502RA CPU features: NMOS