Err, the screen shot you have posted shows a fault with the DRAM providing bits 1, 2, 3, 4, 5, 6, and 7.
With IC25 and IC26 not fitted, but with the ‘upper’ RAM chips still present, they will still respond when the Z80A address any address above 32768, but as they are not being supplied with the address from the Z80A (due to IC25 and IC26 missing), the address they use will be incorrect, as their address inputs are floating.
If you want to correctly disable the ‘upper’ RAM, have a read of what I said elsewhere:
1024MAK wrote:
To disable the "upper" 32k of RAM, take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (
link).
I suggest you make a temporary solder connection with an SPST on/off switch in series. Start with the switch set to on. Then if you get to the copyright screen and BASIC works, after that, you can turn the switch off so that some simple BASIC commands can access and therefore test the upper RAM.
This is done because the ROM code tries to see how much RAM there is at start-up. After this, the BASIC ROM will ignore any RAM that is not present (or in this case, is disabled)(well, unless you reset the limits using CLEAR n). But the BASIC POKE and PEEK commands have no limits and work across the whole Z80 address range.
Double and triple check that you have the correct pin on the correct chip, or you WILL damage something. This temporary modification will disable the "upper" RAM (when the switch is ON), as the CAS signal won't reach the RAM chips.
Note that if a faulty DRAM IC is jamming one of the data lines, this won't really help.
I wrote that assuming the user did not have a diagnostic ROM available. The same principles apply when using a diagnostic ROM, if the switch is made (or you don’t use a switch), the diagnostic ROM should report that the ‘upper’ RAM is not fitted or all of the ‘upper’ RAM is faulty (bits 0 through 7). Or it may report that the machine is a 16K version.
To test the ‘upper’ RAM operate the switch to the open position.
All diagnostic ROMs use a machine code program to test the various addresses of each DRAM chip, and depending on what data is read back, they try to work out what the problem may be. But there is more than one possible cause, so they may sometimes produce misleading results. DRAM chips can fail in various ways. LS TTL logic chips, such as the 74LS157 (74LS257, 74HCT257) multiplexer chips (used for IC25 and IC26) are less complex and are generally speaking, more reliable. Yes, occasionally one will fail.
Mark
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