Difference between revisions of "Next Memory Bank Select"

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{{Port
 
{{Port
 
|Number=$DFFD
 
|Number=$DFFD
 +
|NumberDec=57341
 +
|PortMask=%1101 1111 1111 1101
 
|ShortDesc=Provides additional bank select bits for extended memory.
 
|ShortDesc=Provides additional bank select bits for extended memory.
|PortMask=%1101 1111 1111 1101
+
|Readable=No
|Readable=Yes
+
|Writable=Yes
|Writable=No
 
 
|Subsystem=Memory map
 
|Subsystem=Memory map
 
}}
 
}}
 +
: bit 7 = 1 to set pentagon 512K mode (hard reset = 0) †
 +
: bits 3:0 = most significant bits of the 16K RAM bank selected in {{PortNo|$7FFD}} (soft reset = 0)
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Port 0xDFFD bits 3:0 are combined as the highest bits with the bank selected in port 0x7FFD to form a 7-bit bank number.
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Bit 7 can only be modified when pentagon timing is not active.
 +
 +
† Pentagon 512K mode is applied only when the machine is set to pentagon timing ({{NextRegNo|$03}}). In this mode the selected 16K bank comes from port 0x7ffd {bits 7:6, bits 2:0}

Latest revision as of 11:45, 4 May 2020

Number $DFFD
Decimal 57341
Short desc. Provides additional bank select bits for extended memory.
Bit Mask %1101 1111 1111 1101
Readable No
Writable Yes
Subsystem Memory map
bit 7 = 1 to set pentagon 512K mode (hard reset = 0) †
bits 3:0 = most significant bits of the 16K RAM bank selected in Memory Paging Control ($7FFD / 32765) (soft reset = 0)

Port 0xDFFD bits 3:0 are combined as the highest bits with the bank selected in port 0x7FFD to form a 7-bit bank number.

Bit 7 can only be modified when pentagon timing is not active.

† Pentagon 512K mode is applied only when the machine is set to pentagon timing (Machine Type Register ($03)). In this mode the selected 16K bank comes from port 0x7ffd {bits 7:6, bits 2:0}