Force Computers SYS68K/CPU-40, Руководство пользователя

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SECTION 8 APPENDIX TO THE INTRODUCTION TO VMEPROM
I-3
I.1.2 Reading Data from the RAM port
The READ command initiates a data exchange between the character oriented RAM port and an
application and the data is transferred from the RAM port to an application.
The Command Control Buffer to read data from the RAM port is structured as described in
Figure 5.
struct _ccb_read_command
{
unsigned long _access_control_flags;
unsigned long _reserved_for_ME_purpose[ 10 ];
unsigned long command;
unsigned char *buffer;
unsigned long count;
unsigned long block_number;
unsigned long read_mode;
unsigned long _remnant[ 48 ];
};
Figure 2: Structure of CCB used to read data from RAM port
The READ command is described in the Application Command Interface Programming Guide.
Special paramters are:
count:
The Management Entity allows only one byte to be read from the RAM port at the time and refuses
any attempt to read more or less than one byte. Thus, the count has to specify always one byte
($1).
block_number:
Because the RAM port is a character oriented device this entry is not considered and should be
cleared.
read_mode:
Each read access to the RAM port is carried out in the
status mode independent of the state of the
WAIT flag. Thus, any attempt to read a byte from the RAM port either returns an available data byte,
or is refused if no data is available. It is recommendable to clear all bits.