Deciphering Uart Monitor's register details

Owned by Gurce Isikyildiz

Jun 12, 2023

2 min read

Many of these details were uncovered from this thread on Discord:

https://discord.com/channels/719326990221574164/925528378663137322/1002125641199669248

I’ll make good use of Dddaaannn’s final summary, and also copy/paste any other highlights we discover along the way.

Example Output

PC   A  X  Y  Z  B  SP   MAPH MAPL LAST-OP In     P  P-FLAGS   RGP uS IO ws h RECA8LHC
E1B2 00 01 18 00 00 01F0 B300 E300 F0FA    00     22 ..E...Z. ...P 18 -  00 - ..c..lhc

Summary

PC: CPU program counter (16-bit)

A: CPU Accumulator

X: CPU X register

Y: CPU Y register

Z: CPU Z register

B: CPU base page (B) register

SP: Stack pointer (16-bit)

MAPH + MAPL: The current MAP configuration
- TODO: Let’s elaborate further on how to decipher this

LAST-OP: The bytes of the last instruction performed by the CPU, in hex

(?) In: "Instruction," a byte value, seems to always be 00

P: CPU status register, in hex

P-FLAGS: A rendering of the CPU status register, as NVEBDIZC

(?) RGP: Four mysterious status bytes, as MRGP; dunno why the M isn't in the legend but there's space for it

uS: Processor state, a byte described here: https://github.com/MEGA65/mega65-core/blob/master/src/vhdl/gs4510.vhdl#L858

Notably, the monitor only uses the high byte of a 16-bit value for this, so I don't know how this translates exactly.

(?) IO: Some kind of "I/O fast mode," either W for write, R for read, or - for none

(?) ws: Wait states (plural), a byte described throughout gs4510.vhdl as wait_states

h: Hypervisor mode, displayed as H for Hypervisor mode or - for user mode; observed to be when running Hypervisor code vs. MEGA65 ROM/RAM

RECA8LHC: ROM banking flags that correspond to $D030, namely:
- bit7: ROM E
- bit6: character set ROM
- bit5: ROM C
- bit4: ROM A
- bit3: ROM 8
- bit2: palette ROM for colors 0-15
- bit1: external genlock enable (possibly? unrelated to ROM banking?)
- bit0: extended color RAM mapping into $DC00-$DFFF.

Digging for more details

Much of the detail defining the behaviour of the r command in the uart monitor (used to display these register details) is defined in the following file:
- https://github.com/MEGA65/mega65-core/blob/development/src/monitor/monitor.a65

history_msg:
      .byte       13,10,"PC   A  X  Y  Z  B  SP   MAPH MAPL LAST-OP In     "
      .byte       "P  P-FLAGS   RGP uS IO ws h RECA8LHC",13,10,0