Tom Marchant wrote:
FWIW, I have been happily using 90x142 since you mentioned it. I got to
wondering how big I could go, so I tried 120x160 and got:
ISPI001 Invalid screen size. The width cannot exceed 160 and the depth
cannot exceed 62
I have determined that 102x160 works and 103x160 does not.
115x142 works and 116x142 does not.
102x160 is 16,320 bytes.
115x142 is 16,330 bytes.
It seems that ISPF allows 16K (16384 bytes) for the screen buffer.
Right. There are three addressing schemes for 3270 buffer addresses:
o 12-bit addressing. The original "hokey" method in which only the
low-order six bits in each byte are used for addressing and the upper
bits are set as necessary to translate the two-byte value into "valid"
characters. The scheme can address only up to 4096 bytes.
o 14-bit addressing. The low order 14-bits of the address are used for
addressing. The two upper bits must be zero. This scheme is handy
because it can easily coexist with 12-bit addressing. Smaller devices
will always send 12-bit addresses, where the upper two bits are
guaranteed to be non-zero, and larger devices will always send 14-bit
addresses where the upper two bits are guaranteed zero. Code can easily
test the upper two bits to understand what kind of address has been sent
by the device. This scheme can address only up to 16K bytes. This seems
to be the most popular addressing scheme for "large screen savvy"
products. Apparently, it's the one used by ISPF.
o 16-bit addressing. All sixteen bits are used for addressing.
Coexistence with 12-bit addressing is harder because you must test an
indicator in a control block somewhere to tell you whether the device is
using 16-bit addresses. You cannot test the top two bits of the address
to see if 12-bit addresses are in use. This scheme can address up to 64K
bytes--the maximum supported by 3270.
--
Edward E Jaffe
Phoenix Software International, Inc
5200 W Century Blvd, Suite 800
Los Angeles, CA 90045
310-338-0400 x318
edja...@phoenixsoftware.com
http://www.phoenixsoftware.com/
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