The BDOS portion of CPM2.2 allows programs to access the services provided by the CPM operating system. The BDOS provided thirty eight service functions for programs running under CPM2.2. A program accesses the BDOS using an 8080 jump instruction located at address 0005 in memory. To request a BDOS service the program uses an 8080 CALL instruction to memory address 0005. On entry to the call the CPU "C" register contains the BDOS service request number and the "DE" CPU register is used to pass a 16 bit parameter or the "E" register is used for an 8 bit parameter. When the call returns to the program results will be passed to the program in a CPU registers "A" and/or "HL". Many of the service functions relating to file service and file IO require the memory address of an FCB (File Control Block) structure.
BDOS Overview
I have grouped the services provided by the BDOS into the following categories; system services, serial device services, file services, file I/O services, and disk device services.
The tables below provide a quick reference to the CPM2.2 BDOS services broken down by category,
System Services
| Service | Description | Parameters | Result |
| 0 | Terminate program | n/a | n/a |
| 12 | Get system version | n/a | H = ident L = version |
Serial Services
| Service | Description | Parameters | Result |
| 1 | Console (CON) read byte with echo | n/a | A = value |
| 2 | Console (CON) write byte | E = value | n/a |
| 3 | Reader (RDR) read byte | n/a | A = value |
| 4 | Punch (PUN) write byte | E = value | n/a |
| 5 | List (LST) write byte | E = value | n/a |
| 6 | Direct console (CON) read/write | E = 255 read E 255 write |
Read A = value or 0 for no data |
| 7 | Get IOBYTE | n/a | A = IOBYTE |
| 8 | Set IOBYTE | E = value | n/a |
| 9 | Console (CON) write "$" terminated 7 bit string | DE = address of string | n/a |
| 10 | Console (CON) read line into buffer. First byte max size, second byte length read. Third byte first byte of string. | DE = buffer address | n/a |
| 11 | Console (CON) check if byte can be read | n/a | A = 0 not ready |
File Services
| Service | Description | Parameters | Result |
| 15 | Open existing file referenced by FCB | DE = FCB address | A = 0..3 Ok A = 255 failed |
| 16 | Close file open for output referenced by FCB |
DE = FCB address | A = 0..3 Ok A = 255 failed |
| 17 | Search for first file match referenced by FCB |
DE = FCB address | A = 0..3 Ok A = 255 failed |
| 18 | Find next file match referenced by FCB |
DE = FCB address | A = 0..3 Ok A = 255 failed |
| 19 | Delete File referenced by FCB |
DE = FCB address | A = 0..3 Ok A = 255 failed |
| 22 | Make and open new file referenced by FCB |
DE = FCB address | A = 0..3 Ok A = 255 failed |
| 23 | Rename file, data area of FCB contains new name | DE = FCB address | A = 0..3 Ok A = 255 failed |
| 30 | Set file attributes FCB contains new attributes |
DE = FCB address |
n/a |
| 32 | Get / Set the User Code Area | E = 255 Get E = 0..15 Set |
A = User |
File I/O Services
| Service | Description | Parameters | Result |
| 20 | Sequential read file referenced by FCB |
DE = FCB address | A = 0 Ok A 0 failed A = 255 EOF |
| 21 | Sequential write file referenced by FCB |
DE = FCB address | A = 0 Ok A = 1 no dir A = 2 full disk |
| 26 | Set file transfer buffer address | DE = buffer address | n/a |
| 33 | Direct read file | DE = FCB address | A = 0 Ok A 0 failed |
| 34 | Direct write file | DE = FCB address | A = 0 Ok A 0 failed |
| 35 | Get file end address | DE = FCB address | A = Ok A 0 failed |
| 36 | Get direct address | DE = FCB address | A = Ok A 0 failed |
| 40 | Direct write file zero fill | DE = FCB address | A = Ok A 0 failed |
Disk Services
| Service | Description | Parameters | Result |
| 13 | Reset all disks | n/a | n/a |
| 14 | Select Drive | E = drive | n/a |
| 24 | Get active drive map | n/a | HL = one bit for each of 16 drives |
| 25 | Get default drive number | n/a | A = drive |
| 27 | Get allocation vector | n/a | HL = address of drive allocation vector |
| 28 | Protect current drive Make drive read only |
n/a | n/a |
| 29 | Get read only map | n/a | HL = one bit for each of 16 drives |
| 31 | Get active disk disk parameter block (DPB) | n/a | HL = address of DPB |
For more information about a group of service functions select the table name link for the service group desired.
