390_I_O

=390 / I/O=

When the Hercules ARCHMODE config variable is set to either ESA/390 or ESAME ( or z/Arch), the emulator executes I / O using "390 I/O" mode.

As opposed to 370 I/O, the mechanism is quite different, while sharing some basic concepts.

In both 390 and 370 modes, the instructions used to initiate I/O operations work indirectly with the I/O device.

While a CCW is the eventual target that is passed on to the channel, in 370 mode, the address of the CCW is "hard coded" to address 72 (x'48), while in 390 mode, the CCW address is contained in another variable structure called the ORB

Also, instead of SIO as used in 370, an SSCH (Start Subchannel) instruction is used, with the operand field of the SSCH instruction containing the address of the ORB to be used.

The ORB in turn, contains the device address, as well as the CCW address.