ЫЫЫЫ XE-Bank Programmer's ManualЫ Ы Copyright 1992 by Duane TribeЫ July 19, 1992Ы Ы Ы OVERVIEWЫ Ы This package provides CC65 programs access to the 64K of extendedЫ memory in an Atari 130XE or compatibly-expanded Atari computer as anЫ alternate memory space. Although the access may be slow, the routinesЫ enable C programs to perform many new and larger tasks.Ы Ы The routines are grouped into several object files; see the User'sЫ Manual for descriptions of each routine.Ы Ы Ы XE MEMORY LAYOUTЫ Ы The XE memory is organized as 4 banks of 16K bytes each. To accessЫ one of the banks, main memory from 0x4000 to 0x7FFF is replaced by theЫ XE memory bank. This memory range is the "access window".Ы Ы The routines are limited to just 64K because CC65 cannot directlyЫ manipulate values larger than 16 bits.Ы Ы The bits in the address are interpreted as:Ы Ы BBAA AAAA AAAA AAAAЫ Ы whereЫ BB - Bank number (0..3)Ы AA AAAA AAAA AAAA - Address in bank (0..16383)Ы Ы This is converted to a bank selection byte:Ы Ы 1110 BB11Ы Ы and an address in main memory:Ы Ы 01AA AAAA AAAA AAAAЫ Ы Ы DYNAMIC MEMORYЫ Ы Memory blocks are allocated from a linked list of free blocks. TheЫ word _freelis points to the first free block. The list thenЫ progresses linearly up through the XE memory space.Ы Ы Blocks are allocated by finding the first free block that is as largeЫ as or larger than the requested block. If the block that is chosen isЫ at least 4 bytes larger than the request, the excess bytes are splitЫ off as a new free block.ЫЫЫЫЫЫЫ 1ЫЫЫЫЫЫЫ Ы Each block, either allocated or free, is composed as:Ы Ы +-- Address of blockЫ |Ы vЫ SS NN...Ы Ы whereЫ SS - Size of the block (2 to 65534)Ы NN - Address of next free block (if in the free list)Ы NN... - Data bytes (if the block is in use)Ы Ы Every block has a "size" word (SS) just before the block's address. Ы If the block is in the free list, the first two data bytes (NN) areЫ used as a pointer to the next free block. A NULL pointer indicatesЫ the end of the free block list.Ы Ы Thus, there is a 2-byte overhead per block, with a minimum of 4 bytesЫ total used per block. The size of the block is only limited by howЫ much contiguous unused memory is available.Ы Ы The block release routine, xfree(), automatically merges adjacent freeЫ blocks. But as blocks are allocated and only some released, the XEЫ memory space may become fragmented. The user can reduce fragmentationЫ by allocating long-term use memory blocks first.Ы Ы Ы 6502 SUBROUTINESЫ Ы This is a set of machine language subroutines used by the C routinesЫ to access the XE memory.Ы Ы The calling routine must swap in the appropriate XE bank into theЫ access window as needed and return main memory to that window beforeЫ exiting. These subroutines must not be called by routines under theЫ access window.Ы Ы The routines access XE memory bytes through two sets of zero-pageЫ pointers and bank numbers: xbank1 and xaddr1, or xbank2 and xaddr2. Ы The bytes xbank1 and xbank2 are the bank number along with other bitsЫ for the PIA port. The 2-byte pointers xaddr1 and xaddr2 are theЫ corresponding addresses within the access window.Ы Ы These symbols are not resolved by the linking process:Ы Ы xbase = $96Ы xaddr1 = $98Ы xbank1 = $9AЫ xaddr2 = $9BЫ xbank2 = $9DЫ Ы Each of the routines returns with the 6502 A and X registersЫЫЫЫЫЫЫ 2ЫЫЫЫЫЫЫ scrambled. The value in the Y register remains intact.Ы Ы xpointЫ Ы ldy #[addr flag]Ы jsr xpointЫ Ы Convert 16-bit XE memory address in xbase to a bank number and accessЫ window address. If Y (the 6502 Y register) is 0, the data are storedЫ at xbank1 and xaddr1; if Y is 3, then xbank2 and xaddr2 are usedЫ instead.Ы Ы Does not actually bring the bank into view; see Access Window ControlЫ below.Ы Ы xincrЫ Ы ldy #[addr flag]Ы jsr xincrЫ Ы If Y is 0, increments xaddr1,xaddr1+1 to point to next byte in theЫ XE-bank access window. If the address is beyond the access window,Ы the address is wrapped back to the start of the window and xbank1 isЫ incremented. If Y is 3, then xbank2 and xaddr2 are used instead.Ы Ы If the address crosses a 16K boundary, the new bank will be placed inЫ the access window, even if main memory was in the access window.Ы Ы xdecrЫ Ы ldy #[addr flag]Ы jsr xdecrЫ Ы If Y is 0, decrements xaddr1,xaddr1+1 to point to next byte in theЫ XE-bank access window. If the address is beyond the access window,Ы the address is wrapped back to the end of the window and xbank1 isЫ decremented. If Y is 3, then xbank2 and xaddr2 are used instead.Ы Ы If the address crosses a 16K boundary, the new bank will be placed inЫ the access window, even if main memory was in the access window.Ы Ы Access Window ControlЫ Ы To swap the bank in xbank1 into the access window, useЫ Ы lda xbank1Ы sta $D301Ы Ы To swap the bank in xbank2 into the access window, useЫ Ы lda xbank2Ы sta $D301Ы ЫЫЫЫЫЫЫ 3ЫЫЫЫЫЫЫ To return main memory to the access window, useЫ Ы lda #$F3Ы sta $D301Ы Ы ExampleЫ Ы To reference the byte at 0x1234 in XE memory:Ы Ы lda #$34Ы sta xbaseЫ lda #$12Ы sta xbase+1Ы Ы ldy #0Ы jsr xpointЫ Ы lda xbank1Ы sta $D301Ы Ы lda (xaddr1),yЫ Ы Do not reference subsequent bytes by incrementing Y unless you can beЫ sure that the target address will not fall outside the access window. Ы Instead, use xincr:Ы Ы jsr xincrЫ lda (xaddr1),yЫ Ы xincr will handle the necessary bank swapping when crossing the bankЫ boundaries at 0x0000, 0x4000, 0x8000, and 0xC000.Ы Ы In this example it was not necessary to call xpoint. The xbank1 andЫ xaddr1 values could have been computed by hand and stored directly:Ы Ы lda #$E3 ; bank 0Ы sta xbank1Ы lda #$52 ; $12 into windowЫ sta xaddr1+1Ы lda #$34 ; low byteЫ sta xaddr1Ы Ы Remember to return main memory to the access window before attemptingЫ to access main memory that may reside under the window.Ы ЫЫЫЫЫЫЫЫЫЫЫЫЫЫЫ 4ЫЫЫ