The DMA transfer routine is in bank $15, specifically $15CA8B. If you set a breakpoint here you can see what screens are being loaded to VRAM and where the data is coming from. Here's the routine with some comments:
15/CA8B: A9 80 LDA #$80 ;Set VRAM transfer mode to word-access, increment by 1
15/CA8D: 8D 15 21 STA $2115 ;VRAM Address Increment Value
15/CA90: 9C 0B 42 STZ $420B ;DMA Control Register
15/CA93: A9 01 LDA #$01 ;
15/CA95: 8D 00 43 STA $4300 ;Transfer type
15/CA98: A9 18 LDA #$18 ;($2118?)
15/CA9A: 8D 01 43 STA $4301 ;DMA Channel Destination
15/CA9D: A5 3C LDA $3C ;Bank for Data
15/CA9F: 8D 04 43 STA $4304 ;
15/CAA2: A6 47 LDX $47 ;
15/CAA4: 8E 16 21 STX $2116 ;Address for VRAM Read/Write
15/CAA7: A6 3D LDX $3D ;Source address offset
15/CAA9: 8E 02 43 STX $4302
15/CAAC: A6 45 LDX $45 ;Number of bytes to copy
15/CAAE: 8E 05 43 STX $4305
15/CAB1: A9 01 LDA #$01
15/CAB3: 8D 0B 42 STA $420B
15/CAB6: 6B RTL
Note the zero page data was set up by the caller. This is for FFIIus v1.1
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