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Home CPM 80 CPM BDOS BDOS Overview

BDOS Overview

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 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.
 
 

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.

Last Updated on Saturday, 16 March 2013 06:00