IMS T400
/ 74
8
ProcSpeed-
Select2
ProcSpeed-
Select1
ProcSpeed-
Select0
Processor
Clock
Speed MHz
20.0
22.5
25.0
30.0
35.0
Processor
Cycle
50.0
44.4
40.0
33.3
28.6
Notes
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Not supported
Not supported
Not supported
Not supported
Invalid
Not supported
Invalid
17.5
57.1
Table 3.2
Processor speed selection
3.5
Bootstrap
ThetransputercanbebootstrappedeitherfromalinkorfromexternalROM.Tofacilitatedebugging,
Boot-
FromROM
maybedynamicallychangedbutmustobeythespecifiedtimingrestrictions.Itissampledonce
only by the transputer,before the first instruction is executed after
Reset
is taken low.
If
BootFromROM
is connected high (e.g. to
VDD
) the transputer starts to execute code from thetop two
bytes in externalmemory,at address#7FFFFFFE. Thislocationshould containa backwardjump toa pro-
gram in ROM. Following this access,
BootFromROM
may be taken low if required. The processor is in
the low priority state, and the
W
register points to MemStart (page 4).
If
BootFromROM
is connected low (e.g. to
GND
) the transputer will wait for the first bootstrap message
toarriveonanyone ofitslinks.Thetransputerisreadytoreceivethefirstbyteonalinkwithintwoprocessor
cycles
TPCLPCL
after
Reset
goes low.
If the first byte received (the control byte) is greater than 1 it is taken as the quantity of bytes to be input.
The followingbytes,tothat quantity,are then placedin internalmemory startingat locationMemStart.Fol-
lowing reception of the last byte thetransputer willstart executing code at MemStartas a low priority pro-
cess.
BootFromROM
may be taken high after reception of the last byte, if required. The memory space
immediately abovethe loadedcodeisused asworkspace.Abytearrivingontheotherlinkafterthecontrol
byte has beenreceived and on thebootstrapping linkafter the last bootstrapbyte, will be retainedand no
acknowledge will be sent until a process inputs from them.
3.6
Peek and poke
Any locationin internalorexternal memorycan beinterrogated and altered whenthetransputer is waiting
for a bootstrap fromlink. If the control byte is 0 then eight more bytes are expectedon the same link. The
first four byteword istaken asan internalor externalmemory addressat whichto poke (write) thesecond
four byteword.Ifthecontrol byteis 1 thenextfour bytesare usedas theaddressfromwhich topeek (read)
a word of data; the word is sent down the output channel of the same link.
Following sucha peekorpoke,thetransputer returnstoits previouslyheld state.Any numberofaccesses
may be madein this way untilthe control byte is greaterthan 1, when the transputer willcommence read-
ing its bootstrap program. Either link can be used, but addresses and data must be transmitted via the
same link as the control byte.