Four. File access

4.1 Access model

A model diagram of file access is shown in Figure 4.1.

Figure 4.1 Access function model

Transferring data from the CD-ROM to the CD buffer is called "reading", and transferring data from the CD buffer to the host area is called "extracting".
By using GFS_Fread and GFS_NwFread, the application can transfer data from the CD-ROM without being aware of the reading process.
Use GFS_NwCdRead if you also want the application to control the read process.

4.2 Access pointer

The access pointer is updated by reading, so it moves on a sector-by-sector basis.
Figure 4.2 shows the movement of the access pointer when the process of reading 5000 bytes into the host area buf is executed by executing the following formula. After execution, the access pointer has moved from AP1 to AP2.

GFS_Fread (gfs, 3, buf, 5000);

Figure 4.2 Moving the access pointer

Table 4.1 shows the sector sizes for each file type.

Table 4.1 Sector length for each file type

file category	Sector length (byte)
CD-ROM Mode1	2048
CD-ROM Mode2 Form 1 only	2048
CD-ROM Mode2 Form2 only	2324
CD-ROM Mode 2 mixed	Indefinite
DOS file	2048
Memory file	2048

Reading is done in sector units, while fetching is done in 4-byte units.

4.3 Parameters managed for each file

Of the parameters managed by the library for each opened file, the application can change five parameters. These parameters are shown in Table 4.2.

Table 4.2 Parameters of each file

Parameters	explanation	Function to change	initial value
Access pointer	Offset on the file to start reading (unit is sector)	GFS_Seek	0
Eject mode	After removing the CD Specifies whether to delete or keep the sector data in the buffer	GFS_SetGmode	GFS_GMODE_ERASE
Transfer mode	Specify the device to perform the retrieval process	GFS_SetTmode	GFS_TMODE_CPU
Read parameters	Maximum number of sectors to be transferred in one read process	GFS_SetReadPara	GFS_RPARA_DFL
Extraction parameters	Number of sectors to be transferred in one retrieval process	GFS_SetTransPara	1

The open file occupies one aperture and one buffer partition, which are the resources of the CD block.