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BIOSDesignSpec.md

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CP/M BIOS Design Specification

Overview

  • 62KB BIOS
  • Written in Z80 mnemonic
  • Mode2 interrupt
  • CON: is supported. LST:, PUN: and RDR: are not supported.
  • DISK I/O
    • 5 drives (A: to E:) is supported.
    • Blocking and Deblocking is suppotred.

Boot Sequence

AVR firmware behaves as below at power-on reset.

  1. CP/M boot sequence is triggered if DIPSW-1 is ON and the microSD Card is mounted.
  2. Read EntryAddress and Length from EEPROM. (1)
  3. Set JP EntryAddress to 0x0000 of SRAM. (2)
  4. Copy BIOS on EEPROM to EntryAddress. (3)
  5. Resaet Z80.
  6. Start TinyMonitor.

Z80 launches CP/M as follows:

  1. Start BIOS.
  2. BIOS BOOT routine requests DISK READ to AVR in order to download CCP+BDOS from EntryAddress.
    AVR reads CCP+BDOS from Track 0 on the microSD Card and copies it to SRAM.
  3. BIOS BOOT routine launches CCP.

BOOT and WBOOT

Both routine loads CP/M(CCP+BDOS) from A: using the DISK READ routine. If loading was a failure, then halt the system. The difference between BOOT and WBOOT is as follows.

BOOT

  • Called only at power-on reset.
  • Z80 interrupt settings, Emulated I/O device initialization.
  • Set initial drive to A:.
  • Start CCP from CCP+3.

WBOOT

  • Restore the drive previously being utilized.
  • Start CCP from CCP+0.

DISK I/O

The logical tracks and sectors specified by BDOS are converted into absolute sector numbers. The definition of absolute sector is as follows. TRK and SEC shall be set in advance with the BIOS calls SETTRK and SETSEC, respectively. Also, SPT is a value defined by DPB (Disk Parameter Block).

  • Logical track number: TRK
  • Logical sector number: SCT
  • Number of sectors per logical track: SPT
  • Absolute sector number: AS = TRK * SPT + SCT

A 512-byte buffer (DMABUF) is reserved on SRAM for data sharing with AVR and is used exclusively by READ and WRITE BIOS calls. Since the logical sector length of CP/M is 128 bytes, it supports Blocking/Deblocking for four sectors, that is, Blocking Factor=4.

READ

When a READ request is received, it checks whether the specified sector exists in DMABUF. If it exists, copy that sector to DMA_ADRS. If it does not exist, request AVR to read 512 bytes. Copy the first 128 bytes (1 sector) to DMA_ADRS. The remaining three sectors are saved as a read-ahead cache.

WRITE

Emulated DISK I/O on AVR allows writing to a disk image file by specifying an arbitrary address and data length. However, microSD cards require writing in 512-byte units from 512-byte sector boundaries, so if the boundaries do not match or the data length is not a multiple of 512, Read/Modify/Write is required. This causes a noticeable write performance reduction, so the BIOS manages the timing of buffers and buffer flushes (write requests to AVR) to write 512 bytes from the 512-byte boundary as much as possible.

When a WRITE request is received, the data is copied to the IDX location defined below.

  • Location of stored buffer: IDX = AS % 4 
          +----------+
IDX=0 | 128bytes |
      +----------+
IDX=1 | 128bytes |
      +----------+
IDX=2 | 128bytes |
      +----------+
IDX=3 | 128bytes |
      +----------+

If this is the first WRITE request after flushing the buffer, also remember IDX and image file write destination address. This will be used during the next buffer flush.

The caller of the WRITE request sets the following Deblocking Code in the C register.

  • C = 0: deferred write
  • C = 1: nondeferred write
  • C = 2: deferred write to the first sector of a new data block

If any or both of the following conditions are met, flush the buffer. Otherwise, increment the number of cached sectors (0-3) and return.

  • C=1
  • IDX=3

When flushing the buffer, write the number of cached sectors x 128 bytes from the stored IDX to the image file write destination address.

SELDSK

Emulated DISK I/O switches the accessed disk image to that of the specified drive.

SETTRK

Just save the logical track number in CURRENT_TRACK_NO. It will be used in the next READ/WRITE.

SETSEC

Just save the logical sector number in CURRENT_SECTOR_NO. It will be used in the next READ/WRITE.

SERDMA

Just save the data transfer address in DMA_ADRS. It will be used in the next READ/WRITE.

SECTRN

It is not supported since interleave is unnecessary for accessing microSD Cards.