Saturday, February 3, 2018

RTC library moved to EPROM, kernel jump table extended.

Hello 8-bit computing enthusiasts.

A little update on the MKHBC-8-Rx project.
Recently having (mostly) completed my home improvement projects, I have some more time to play with my pet project.
I have been mainly working on code, cleaning it up and moving some essential parts to EPROM. It is still a mess, but I think it is good enough to be published, so I put it on github.

I moved assembly code of RTC DS-1685 driver to EPROM, enabled periodic interrupts from RTC and added interrupt routine for RTC. I also expanded kernel jump table and added some new functions to my so called 'enhanced shell', although calling this program a shell is a bit of an overkill. :-)

Below I present the current documentation of the OS / firmware fir this system:

System programming / Operating System of MKHBC-8-Rx is divided into 2 parts:

  • Firmware, which resides in EPROM.
  • System programs, which are loaded to RAM via serial port.

Firmware consists of hexadecimal machine code monitor, hardware drivers code, MOS-6502 mandatory vectors: Reset, NMI, IRQ and Kernel Jump Table.
Firmware is implemented in MOS 6502 assembly language.
Kernel Jump Table is a series of jump commands that redirect to code performing certain system functions.
These jump commands are guaranteed to be always at the same memory locations, so the system programs using them don't have to be rebuilt each time the implementation of firmware is changed and some internal addresses has moved. The jump table is always in the same place jumping from the same locations to the same functions that they should perform, even if the functions themselves change the location in the EPROM due to code refactoring or relocation of binary code.
The new entries can be added to the Kernel Jump Table though, so it is guaranteed to be backwards compatible.

Theory of operation:

When computer is started, the reset circuit holds the reset line low long enough for CPU to initialize, then the CPU performs startup routine which consists of reading the Reset vector from the EPROM memory (which must be at fixed address) and executes the routine pointed by that vector.
The routine contains initialization code for OS and then goes into the eternal loop which sends the output via serial port and expects input on the serial port. By connecting text serial terminal device configured to be the same speed as computer's UART speed, it is possible to interface with the computer to send commands and receive output.

The command UI is very simple and consists of very rudimentary hexadecimal machine code monitor which allows to read/dump values in computer's memory, write/modify values in Random Access Memory and execute code at provided address.
This UI is rudimentary but sufficient for entering code into computer's RAM and executing it.

Programming API / Kernal Jump Table:

Address (hex)
RegB, RegA, RegXB, RegXA
RegC in Acc
Initialize RTC chip.
Data is returned via hardware stack. Calling subroutine is responsible for allocating 8 bytes on stack before calling this function. Clock data are stored in following order below the subroutine return address: seconds, minutes, hours, dayofweek, date, month, year, century. Valid return data on stack only if Acc > 0 (Acc = # of bytes on stack). Calling subroutine still should de-allocate stack space by calling PLA x 8 after reading or discarding returned data.
Read RTC clock data.
Parameters are passed via hardware stack: seconds, minutes, hours, day of week, day of month, month, year, century. Calling subroutine is responsible for allocating 8 bytes on stack and filling the space with valid input data before calling this function. Calling subroutine is also responsible for freeing stack space (PLA x 8).
Set date/time of RTC chip.
Parameters are passed via hardware stack: seconds, minutes, hour. Calling subroutine is responsible for allocating space on stack and filling it with valid input data before calling this function.  Calling subroutine is also responsible for freeing stack space (PLA x 3).
Set time of RTC chip.
BankNum, RamAddr, RamVal
Store a value in non-volatile RTC memory bank.
BankNum, RamAddr
value in Acc
Read value from non-volatile RTC memory bank.
PromptLine (contains hexadecimal address range)
n/a (output)
Machine code monitor function - read memory.
PromptLine (contains hexadecimal address and values)
n/a (memory is modified)
Machine code monitor function - write memory.
PromptLine (contains hexadecimal address)
n/a (code is executed)
Machine code monitor function - execute code in memory.
Character code in Acc
Standard I/O function - get character.
Character code in Acc.
n/a (output)
Standard I/O function - put/print character.
n/a (input)
PromptLine, PromptLen
Standard I/O function - get string.
n/a (output)
Standard I/O function - put/print string.
Banked RAM bank # in Acc. (0..7)
n/a (selects RAM bank, updates shadow register in RAM)
Banked RAM bank selection.

Disable interrupts before calling any RTC function:
<call to RTC API>

Registers, buffers, memory:

RTC RAM shadow:
               RegB        = $F6
               RegA        = $F7
               RegXB       = $F8
               RegXA       = $F9
               RegC        = $FA
               Temp        = $FB
               BankNum     = $FC
               RamAddr     = $FD
               RamVal      = $FE

Uart Queues (after stack)
               UartTxQue   = $200          ; 256 byte output queue
               UartRxQue   = $300          ; 256 byte input queue

          MOS Prompt variables
               PromptLine  = $80           ; Prompt line (entered by user)
               PromptMax   = $50           ; An 80-character line is permitted ($80 to $CF)
               PromptLen   = $D0           ; Location of length variable

          MOS I/O Function variables
               StrPtr      = $E0           ; String pointer for I/O functions

 Other variables:
 Timer64Hz   = $E2            ; 4-byte (32-bit) counter incremented 64 times / sec
   ; $E2,$E3,$E4,$E5 (unsigned long, little endian)

          Customizable jump vectors
          Program loaded and run in RAM can modify these vectors
          to drive custom I/O console hardware and attach/change
          handler to IRQ procedure. Interrupt flag should be
          set before changes are applied and cleared when ready.
          Custom IRQ handler routine should make a jump to standard
          handler at the end. Custom I/O function routine should

          end with RTS.

               StoreAcc  =  $11          ; Temporary Accumulator store.
               IrqVect   =  $0012        ; Customizable IRQ vector
               GetChVect =  $0014        ; Custom GetCh function jump vector
               PutChVect =  $0016        ; Custom PutCh function jump vector
               GetsVect  =  $0018        ; Custom Gets function jump vector
               PutsVect  =  $001a        ; Custom Puts function jump vector

I/O space / address range:

$C000 .. $C7FF, 8 pages (8 x 256 bytes):

Internal (non-buffered) I/O bus:

$C000 .. $C0FF - slot 0 (RAM bank switching register)
$C100 .. $C1FF - slot 1 (RTC registers)
$C200 .. $C2FF - slot 2 (Reserved for Prioritized Interrupt Controller)
$C300 .. $C3FF - slot 3 (Reserved for built in I/O parallel interface PIA or VIA)

External (buffered/expansion) I/O bus:

$C400 .. $C4FF - slot 4 (UART)
$C500 .. $C5FF - slot 5
$C600 .. $C6FF - slot 6
$C700 .. $C7FF - slot 7

RAM bank switching.

$00 .. $07
 Banked memory:
$8000 .. $BFFF
 Bank number RAM register:

Memory map:

$0000 - $7FFF: Base RAM, 32 kB.
$6000 - $7FFF: Optional Video RAM, 8 kB.
$8000 - $BFFF: Banked RAM, 16 kB space x 8 banks = 128 kB.
$C000 - $C7FF: I/O space, 8 slots x 256 Bytes = 2 kB.
$C800 - $FFFF: EPROM, 14 kB.

System programs:

System programs currently consist only one - enhanced shell.
It is written in C and compiled with CC65.
Programs written in C (CC65) or CA65 assembly for MKHBC-8-Rx computer / MKHBC OS use library written in C and assembly languages which implements standard C library (CC65), I/O console and RTC functions and are compiled into library archive mkhbcos.lib.
Corresponding C header files are:

  • mkhbcos_ansi.h - ANSI terminal API
  • mkhbcos_ds1685.h - DS1685 RTC API
  • mkhbcos_lcd.h - standard LCD 16x2 API
  • mkhbcos_ml.h - machine code monitor API
  • mkhbcos_serialio.h - serial I/O API

That's all for today.
Thank you for visiting my blog.

Marek K. 2/4/2018.