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. | ||
|Readable=No | |Readable=No | ||
|Writable=Yes | |Writable=Yes | ||
|Subsystem=Memory map | |Subsystem=Memory map | ||
| − | |||
}} | }} | ||
: bit 7 = 1 to set pentagon 512K mode (hard reset = 0) † | : 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}} | + | : bits 3:0 = most significant bits of the 16K RAM bank selected in {{PortNo|$7FFD}} (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. | 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 ({{NextRegNo|$03}}). In this mode the selected 16K bank comes from port 0x7ffd {bits 7:6, bits 2:0} | † 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}