Introducing jzIntv's Debugger

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jzIntv offers a simple command-line oriented debugger. It should be familiar in style to anyone who has used the Apple ][ Monitor or DOS's DEBUG.EXE.

Debugger Overview

Invoking the Debugger

To invoke the debugger, add the "-d" flag to jzIntv's command line. For example, using "hello2.rom" from the Hello World Tutorial:

    jzintv -d hello2.rom

This will invoke jzIntv, and present you with a prompt:

jzintv:  Initializing Master Component and peripherals...
gfx:  Searching for video modes near 320x200x8 with:
gfx:      Hardware surf, Double buf, Sync blit, Software pal, Windowed
gfx:  Selected:  320x200x8 with:
gfx:      Software surf, Single buf, Sync blit, Hardware pal, Windowed
snd:  buf_size: wanted 2048, got 2048
ay8910:  Automatic sliding-window setting: 10
CP-1610          [0x0000...0x0000]
PSG0 AY8914      [0x01F0...0x01FF]
[Graphics]       [0x0000...0x0000]
[Sound]          [0x0000...0x0000]
Scratch RAM      [0x0100...0x01EF]
System RAM       [0x0200...0x035F]
EXEC ROM         [0x1000...0x1FFF]
Pad Pair 0       [0x01F0...0x01FF]
STIC             [0x0000...0x007F]
STIC             [0x4000...0x403F]
STIC             [0x8000...0x803F]
STIC             [0xC000...0xC03F]
STIC (BTAB)      [0x0200...0x02EF]
STIC (GRAM)      [0x3000...0x3FFF]
[Event]          [0x0000...0x0000]
[Rate Ctrl]      [0x0000...0x0000]
ICart   [R   ]   [0x5000...0x50FF]
CP-1610 Snoop    [0x0200...0x035F]
[Debugger]       [0x0000...0xFFFF]
 0000 0000 0000 0000 0000 0000 0000 1000 -------Q  JSRD R5,$1026             0

Most of this output is jzIntv's initialization. The last portion is the debugger prompt.

The Debugger Prompt

 0000 0000 0000 0000 0000 0000 0000 1000 -------Q  JSRD R5,$1026             0

The prompt shown above is the debugger's input prompt. From here, you can tell the debugger what to do next. Before each prompt, jzIntv reports specific information about the state of the machine. The following diagram illustrates:

Jzintv debugger status line.png

R0 through R7 are the CPU's 8 registers. Each register is 16 bits wide. jzIntv's debugger shows their values in hexadecimal.

The "flags" field shows what CPU flags are currently set. jzIntv tracks 8 separate flags:

  S   Sign Flag
  C   Carry Flag
  O   Overflow Flag
  Z   Zero Flag
  I   Interrupt Enable Flag
  D   Double Byte Data Flag
  i   Previous instruction was interruptible

When a flag is set, jzIntv shows the letter for that flag. When the flag is clear, jzIntv shows a dash. This makes it easy to see what flags are currently set, without having to remember their exact order. The last flag position is special. It shows the current interrupt status:

  q  Interrupt asserted
  b  BUSRQ asserted
  Q  Interrupt being taken
  B  CPU halted by BUSRQ

In the example above, jzIntv shows the CPU as taking an interrupt. Really, it's coming out of reset, which is similar. Don't worry too much about interrupts for the moment.