Skip to content

Latest commit

 

History

History

rom

README.md

Kaypro ROM disassembled and commented

Versions

ROMs analysed

Both ROMs are very similar, 81-232 changes the welcome string and adds support for double sided disks, DSDD.

The files 81-149c.s, and 81-232.s can be assembled with z80asm to generate binaries identical to the initial ROMs. See verify.sh

Analysis

The ROM has these main sections:

  • entry points (DONE)
  • initialization (DONE)
  • loading the OS from disk (DONE)
  • disk read and write (DONE)
  • keyboard, serial and parallel ports usage (DONE)
  • formatted output to the screen (DONE)

Entry points

They are very similar but not identical to the CP/M BIOS entrypoints. The CP/M BIOS entry points enable the ROM and do not need to do much more than calling the proper Kaypro ROM entry points besides looking IOBYTE to see the device mappings.

The entry point names have been taken from the KayPLUS manual.

Initialization

On initialization the system is configured:

  • the serial, keyboard, parallel and system bits are configured accessing the ports of the Z80-SIO and Z80-PIO
  • the screen is cleared, the cursor moved home, esc mode is terminated and greek alphabet is disabled
  • the floppy disk variables are reset
  • the disk parameter header and blocks and the single density translation table is moved to upper memory to be accessible by the CP/M BDOS when the ROM is paged out.
  • routines to move blocks of memory or read and write block to or from disk is move to upper memory to be able to move, load and save blocks of RAM that share addresses with the ROM area
  • the welcome string is shown

After that, the disk can be booted

Loading the OS from disk

To load the OS, the first sector 0 of track 0 of disk A is loaded. That sector has the information of how many more sectors have to be read, where they have to be loaded and where to jump execution to start the OS. Note that the code only works if booting from a double density disk.

Floppy disk read and write

The entry points comply with the CP/M BIOS specification. Kaypro supports single density (FM) and double density (MFM) disks. When a disk is selected, the ROM tries to read in double density first; if this fails, it tries single density. It stores the density for next use of the disk.

The code for single density is similar to the Appendix B of the CP/M 2.2 Alteration Guide

For double density, the ROM has to do sector blocking and deblocking with code similar to Appendix G of the CP/M 2.2 Alteration Guide

Keyboard, serial and parallel usage

Basic entrypoint to input, output and status on the ports

Formatted output

Code to output characters to the screen by writting to the video memory. It does cursor management, process of control chars and escape sequences.

The control chars processed are: 0xa, 0xd, 0x8, 0xc, 0xb, 0x18, 0x17, 0x1a and 0x1e. The escape commands processed are: G, A, R, E and =.

If greek mode is set (with ESC-G), lowercase characters are shown as greek letters.

Changes in 81-232

The code if similar to the 81-149c ROM with the following differences:

  • The welcome message says "Kaypro" instead of "Kaypro II"
  • Support for double sided doble density disks (DSDD)
  • The code goes 144 bytes beyong the 2KB limit and needs a 4KB ROM. All the reamining spce is filled with FF.
  • The PIO-2A bit 6 is configured as output.

To support the DSDD disks, a new disk parameter block. This block is not copied to upper RAM as the SSSD and SSDD blocks were. Instead the upper RAM copy of SSDD is replaced by the disk parameter block for DSDD when needed. Also, the previously unused system bit 2 is used to select single side or double side mode.

On the ROM 81.149c, the current track of both drives is stored on two variables. Also, the density detected for the current disk on each drive is stored as an aditional 16th byte of the disk parameter header. On 81-232, there is a need to store as well if the disk is double sided. Current track, density and sides are now stored per drive in the variables disk_active_info_drive_a and disk_active_info_drive_b. It is copied back an forth to disk_active_info as A: or B: is selected.

Memory map

For 81-141c:

- 0x0000-0x07ff: ROM

- 0xe400-0xfbff: Area to load OS from disk

- 0xfa00-0xfa7f: First boot sector

- 0xfc00-0xfc15: Work variables for the floopy R/W
- 0xfc16-0xfe15: 512 bytes to buffer double density sectors
- 0xfe16-0xfe19: More work variables for the floopy R/W
- 0xfe1a-0xfe29: CSV_0 Scrathpad for change disk check, drive 0
- 0xfe2a-0xfe42: ALV_0 Scrathpad for BDOS disk allocation, drive 0
- 0xfe43-0xfe52: CSV_1 Scrathpad for change disk check, drive 1
- 0xfe53-0xfe6b: ALV_1 Scrathpad for BDOS disk allocation, drive 1
- 0xfe6c-0xfe70: Work variables for the console output
- 0xfe71-0xfe81: Drive A disk parameter header
- 0xfe82-0xfe92: Drive B disk parameter header
- 0xfe93-0xfea1: Single density disk parameter block 
- 0xfea2-0xfeb0: Double density disk parameter block 
- 0xfeb1-0xfecc: Single density sector translation table
- 0xfecd-0xff53: Disk R/W code relocated to upper memory
- 0xff54-0xff72: BLANK?
- 0xff73-0xfff3: Scratchpad for BDOS directory operations